נשים
הלכות יבום וחליצה
פרק ג
Nashim
Yibum and Chalitzah
Chapter 3

Halacha

הלכה א
הָאוֹמֵר זֶה בְּנִי אוֹ שֶׁאָמַר יֵשׁ לִי בָּנִים הֲרֵי זֶה נֶאֱמָן וּפוֹטֵר אֶת אִשְׁתּוֹ מִן הַחֲלִיצָה וּמִן הַיִּבּוּם:
כסף משנה
1.
When a man says: "This is my son," or "I have sons," his word is accepted,1The rationale for this ruling is the principle of migo. If the husband's statements were untrue, and his intent were merely to free his wife from the obligation of yibbum, he would not have to lie. Instead, he could give her a get (Bava Batra 134b).
See also the Shulchan Aruch (Even HaEzer 156:6) and commentaries, which mention other opinions regarding whether this ruling applies when the prevailing presumption is that the man is childless, when the prevailing presumption is that he has brothers, or when witnesses testify that he has brothers.
and he frees his wife from [the obligation of] yibbum or chalitzah.

הלכה ב
אָמַר זֶה אָחִי אוֹ שֶׁאָמַר יֵשׁ לִי אַחִין אֵינוֹ נֶאֱמָן לֶאֱסֹר אֶת אִשְׁתּוֹ וּלְהַנִּיחָהּ זְקוּקָה לְיָבָם שֶׁהֲרֵי זֶה מִתְכַּוֵּן לְאָסְרָהּ לְאַחַר מוֹתוֹ:
כסף משנה
2.
When2I.e., when the prevailing presumption is that he does not have a brother, and hence his wife would not be obligated to perform yibbum. a man says: "This is my brother," or "I have brothers," his word is not accepted, and his wife does not become forbidden [to others because] she is required to perform yibbum. [We assume that] his intent was to cause his wife to be forbidden [to other men] after his death.

הלכה ג
הָיָה מֻחְזָק שֶׁיֵּשׁ לוֹ אַחִין וְאָמַר בִּשְׁעַת מִיתָתוֹ אֵין לִי אַחִין אֵינוֹ נֶאֱמָן. וְכֵן אִם אָמַר עַל מִי שֶׁהֻחְזַק אָחִיו אֵין זֶה אָחִי אֵינוֹ נֶאֱמָן. לֹא הָיָה מֻחְזָק בְּאַחִין וְיָצָא קוֹל שֶׁיֵּשׁ [שָׁם] עֵדִים שֶׁיָּעִידוּ שֶׁיֵּשׁ לְבַעְלָהּ אַחִין וְהָעֵדִים בִּמְדִינָה אַחֶרֶת. אֲפִלּוּ אָמַר הוּא בִּשְׁעַת מִיתָתוֹ אֵין לִי אָח הֲרֵי זוֹ חוֹשֶׁשֶׁת וְתַמְתִּין עַד שֶׁיָּבוֹאוּ הָעֵדִים שֶׁאָמְרוּ וְיִשְׁאֲלוּ:
כסף משנה
3.
When the prevailing presumption is that [a man] has brothers and he says at the time of his death,3Even though the statements are being made at the time of the person's death, when it is unlikely that he would lie, we still suspect that he is speaking falsely to spare his wife the difficulty of yibbum or chalitzah. "I do not have brothers," his word is not accepted.4One might assume that, as in Halachah 1, the husband's statements would have been accepted based on the principle of migo. For in this instance as well, the husband could free his wife from yibbum by giving her a get. Nevertheless, our Sages explain that the principle of migo does not have the power to counteract a prevailing presumption.
Still, even if the prevailing presumption is that a person has brothers, his word is accepted if he says that he has sons. The rationale is that, in this instance, his statements are not contradicting the prevailing presumption, but rather introducing a new factor (Beit Shmuel 157:9).
Similarly, if he says that a person presumed to be his brother is not his brother, his word is not accepted.
If there was no prevailing presumption that he has brothers, but a rumor becomes circulated5There is a difference of opinion among the Rabbis whether or not this rumor has to be substantiated in a court. The Maggid Mishneh interprets the Rambam's opinion as being that even a rumor that is not substantiated should be taken into consideration. Needless to say, in the contemporary era, when communications have advanced and travel is easier, this question is far less significant than in previous ages. that there are witnesses who will testify that he has brothers, but these witnesses are overseas, the woman must take this factor into consideration and wait until the witnesses who were mentioned come, and are asked [concerning the matter]. [This ruling applies] even when the husband says, "I do not have a brother" at the time of his death.

הלכה ד
מִי שֶׁזָּנָה עִם אִשָּׁה בֵּין פְּנוּיָה בֵּין אֵשֶׁת אִישׁ וְנִתְעַבְּרָה וְאָמַר זֶה הָעֻבָּר מִמֶּנִּי הוּא וַאֲפִלּוּ הִיא מוֹדָה לוֹ אַף עַל פִּי שֶׁהוּא בְּנוֹ לְעִנְיַן יְרֻשָּׁה הֲרֵי זֶה סָפֵק לְעִנְיַן יִבּוּם. כְּשֵׁם שֶׁזִּנְּתָה עִם זֶה כָּךְ זִנְּתָה עִם אַחֵר. וּמֵאַיִן יִוָּדַע הַדָּבָר שֶׁזֶּה בְּנוֹ וַדַּאי וַהֲרֵי אֵין לוֹ חֲזָקָה. אֶלָּא לְעוֹלָם סָפֵק הוּא וּלְהַחֲמִיר דָּנִין בּוֹ וְחוֹלֶצֶת וְלֹא מִתְיַבֶּמֶת:
כסף משנה
4.
[The following rules apply when] a man engaged in licentious relations with either a single woman or a married woman, she became pregnant, and he claims that he fathered the child. Even if [the mother] acknowledges his statements, and thus the son is considered his with regard to the laws of inheritance,6See Hilchot Nachalot 4:1. the matter is considered to be one of doubt with regard to the laws of yibbum. For just as the woman engaged in relations with him, she could have engaged in relations with another man.7If the woman is married, there is no question regarding the Rambam's ruling, for we presume that by and large, she engaged in relations with her husband. If she is single, the matter is more complex.
In this context, the commentaries draw attention to the Rambam's ruling in Hilchot Terumot 8:14, which states that when a priest had relations with a woman and she became pregnant, she is allowed to partake of terumah for the sake of her child. This indicates that we assume that she was impregnated by that priest.
As indicated by the Rambam's wording in Hilchot Terumot and in Hilchot Issurei Bi'ah 15:17-18, the matter depends on the woman's reputation. If she is known to be promiscuous, we consider the possibility that she was impregnated by another man. If, however, everyone presumes that she engaged in relations only with this man, he is considered to be the father of the child (Maggid Mishneh).
Nevertheless, the Shulchan Aruch (Even HaEzer 156:9) quotes the Rambam's wording without making this distinction. See the Beit Shmuel 156:15.
How is it possible to know with certainty that this is his son? There is no prevailing presumption to that effect.
The matter thus remains one of doubt, and the more stringent perspective should be taken. Hence, his wife [is obligated to] perform chalitzah,8Lest the child have been fathered by another man, and the deceased in fact be childless. and she [is forbidden] to perform yibbum.9Lest the child have been fathered by the deceased, in which case relations with his brother would be prohibited.

הלכה ה
נֶאֱמָן עֵד אֶחָד לְהָעִיד לִיבָמָה שֶׁמֵּת בַּעְלָהּ וּמִתְיַבֶּמֶת עַל פִּיו. אוֹ שֶׁמֵּת יְבָמָהּ אוֹ שֶׁנִּתַּן לְבַעְלָהּ בֵּן, לְהַתִּירָהּ לְזָר. אֲפִלּוּ עֶבֶד אוֹ אִשָּׁה אוֹ עַכּוּ״‎ם מֵסִיחַ לְפִי תֻּמּוֹ מֵעִיד בְּמִיתַת הַיָּבָם כְּמוֹ שֶׁמֵּעִיד בְּאֵשֶׁת אִישׁ לְהַתִּירָהּ כְּמוֹ שֶׁבֵּאַרְנוּ בְּהִלְכוֹת גֵּרוּשִׁין:
כסף משנה
5.
The testimony of one witness is accepted with regard to the death of a woman's husband, and she may perform yibbum on this basis. [Similarly, such testimony is acceptable if he states that] her yavam died,10Rabbenu Asher differs with the Rambam regarding this matter and maintains that the testimony of one witness is not acceptable with regard to the death of a yavam. Although the Ramah (Even HaEzer 158:3) quotes his opinion, many Ashkenazic authorities recommend leniency. (See Beit Shmuel 158:2.) or that her husband fathered a son,11The Ra'avad and Rabbenu Asher differ with the Rambam with regard to this point, for unlike the death of a person, the birth of a child is not a matter that will surely become undeniable public knowledge. The Shulchan Aruch (Even HaEzer 156:8) quotes the Rambam's opinion, while the Ramah follows the other view. on which basis she would be granted permission to marry another person [without chalitzah].
Even the testimony of a servant or a woman, or statements of a gentile made in the midst of conversation are acceptable with regard to the death of a yavam, as they are acceptable with regard to a married woman whose husband has died, to enable her to be permitted to remarry, as mentioned in Hilchot Gerushin.12In Hilchot Gerushin, Chapters 12 and 13, the Rambam explains that the testimony of one witness is accepted in such instances, because:

When the matter can be verified definitively without the testimony of a witness, and the witness cannot justify [his statements] if they are not true - e.g., when one testifies that a person died - the Torah did not necessitate [that the requirements of formal testimony be met in these instances]. For it is unlikely that a witness will testify falsely.
For this reason, our Sages [extended] the leniency with regard to this matter and accepted the testimony of a single witness that is based on the testimony of a maidservant, [testimony] from a written document, and [testimony] that was not investigated by the ordinary process of interrogation. [These leniencies were accepted] so that the daughters of Israel will not be forced to remain unmarried.

הלכה ו
וְחָמֵשׁ נָשִׁים שֶׁאֵין מְעִידוֹת זוֹ לָזוֹ שֶׁמֵּת בַּעְלָהּ כָּךְ אֵין מְעִידִין לָהּ שֶׁמֵּת יְבָמָהּ. וְדִין עֵדוּת זוֹ כְּדִין אוֹתָהּ עֵדוּת לְעִנְיַן עֵדִים שֶׁמַּכְחִישִׁין זֶה אֶת זֶה בְּמִיתַת הַיָּבָם וּלְכָל דָּבָר:
כסף משנה
6.
Just as the statements of five [specific] women are not accepted with regard to testimony regarding the death of a woman's husband,13See Hilchot Gerushin 12:16, which states that the statements of a woman's mother-in-law, the daughter of her mother-in-law, [her husband's] other wife, her yevamah (i.e., the wife of her husband's brother) and her husband's daughter are not accepted with regard to her husband's death, because the prevailing presumption is that they hate each other. so too, the statements of these five women are not accepted with regard to the death of her yavam.
The laws of the testimony governing the death of a yavam are the same as the testimony [governing the death of the woman's husband] with regard to disparities between the statements of the witnesses and all other matters.

הלכה ז
שְׁתֵּי יְבָמוֹת שֶׁבָּאוּ מִמְּדִינַת הַיָּם זוֹ אוֹמֶרֶת מֵת בַּעְלִי וְזוֹ אוֹמֶרֶת מֵת בַּעְלִי. זוֹ אֲסוּרָה מִפְּנֵי בַּעְלָהּ שֶׁל זוֹ וְזוֹ אֲסוּרָה מִפְּנֵי בַּעְלָהּ שֶׁל זוֹ. שֶׁאֵין יְבִמְתָּהּ נֶאֱמֶנֶת לְהָעִיד לָהּ שֶׁמֵּת יְבָמָהּ כְּמוֹ שֶׁבֵּאַרְנוּ:
כסף משנה
7.
[The following rule applies when] two widows, [each one married to one of two brothers,]14I.e., the women are married to two brothers. Thus each one is considered the yevamah of the other. come from an overseas country, this one says: "My husband died," and this one says: "My husband died." Each one is forbidden [to remarry] because of her obligation to the husband of the other one.15I.e., the testimony of each woman is accepted with regard to the death of her own husband. With regard to the death of her yavam, by contrast, the only testimony is that of her yevamah, and that testimony is not accepted. For the testimony of a yevamah is not acceptable with regard to the death of the woman's yavam, as stated [in the previous halachah].

הלכה ח
הָיָה לְאַחַת מֵהֶן עֵד שֶׁמֵּת בַּעְלָהּ. זוֹ שֶׁיֵּשׁ לָהּ הָעֵד עוֹמֶדֶת בְּאִסּוּרָהּ שֶׁאֵינָהּ אֲסוּרָה מִפְּנֵי בַּעְלָהּ אֶלָּא מִפְּנֵי יְבָמָהּ. וְזוֹ שֶׁאֵין לָהּ עֵד מֻתֶּרֶת שֶׁהֲרֵי הֵעִיד הָעֵד שֶׁמֵּת יְבָמָהּ וְהִיא נֶאֱמֶנֶת לוֹמַר שֶׁמֵּת בַּעְלִי:
כסף משנה
8.
If one of these women has a witness who can testify that her husband died, she is still forbidden to remarry - for she is not forbidden to remarry because of [an apprehension that] her own husband [is alive], but rather because of [the apprehension that] her yavam [is alive].16And with regard to the death of her yavam, there is no testimony that is acceptable to the court.
Even if the woman actually remarries, at which point it would be likely to assume that her husband is dead, her yevamah is still prohibited. Our Sages feared that a woman would risk ruin in both this world and the next, in order to cause her foe to be forbidden to her husband.

The woman who does not have a witness supporting her, by contrast, is permitted [to remarry], for a witness has testified that her yavam died, and her own testimony is accepted with regard to the death of her husband.17See Hilchot Gerushin 12:15.

הלכה ט
הָיָה לָזוֹ בָּנִים וְלָזוֹ אֵין בָּנִים זוֹ שֶׁאֵין לָהּ בָּנִים אֲסוּרָה וְזוֹ שֶׁיֵּשׁ לָהּ בָּנִים מֻתֶּרֶת. הָיָה לָהֶן יָבָם אַחֵר כָּאן הֲרֵי זֶה מְיַבֵּם לִשְׁתֵּיהֶן:
כסף משנה
9.
If one of the above-mentioned women has children, and the other is childless,18And neither has witnesses to support her testimony. the one who is childless is forbidden [to remarry],19For she is dependent on the testimony of her yevamah, as above. and the one who has children is permitted.20For her authorization to remarry does not depend on whether or not her yavam is alive.
If there is another living brother to perform yibbum, he should perform yibbum21Or chalitzah. with both of them.

הלכה י
מֵת זֶה הַיָּבָם שֶׁיִּבֵּם אוֹתָן אֲסוּרוֹת לְהִנָּשֵׂא לְזָר כְּשֶׁהָיוּ בַּתְּחִלָּה. נִתְיַבְּמוּ וְנִתְגָּרְשׁוּ הֲרֵי אֵלּוּ מֻתָּרוֹת לְזָר:
כסף משנה
10.
If [in the latter instance], the yavam who married them died, they are forbidden to marry another man as they were originally.22I.e., there were only three brothers. Since the death of the first two brothers was never verified adequately to allow these women to marry freely, the original prohibition applying to them remains in force. If, however, the yavam married them and then divorced them, they are free to marry other men.23For her marriage and divorce releases each of the women from their obligation to their missing yavam.

הלכה יא
אַף עַל פִּי שֶׁהָאִשָּׁה נֶאֱמֶנֶת לוֹמַר מֵת בַּעְלִי וְתִנָּשֵׂא אוֹ תִּתְיַבֵּם אֵין הַיְבָמָה נֶאֱמֶנֶת לוֹמַר מֵת יְבָמִי שֶׁתִּנָּשֵׂא לְזָר הוֹאִיל וְהוּא אִסּוּר לָאו שֶׁמָּא יִהְיֶה קַל בְּעֵינֶיהָ. וְכֵן אֵין הַיָּבָם נֶאֱמָן לוֹמַר מֵת אָחִי שֶׁיְּיַבֵּם אֶת אִשְׁתּוֹ שֶׁמָּא עֵינָיו נָתַן בָּהּ. וְאֵין הָאִשָּׁה נֶאֱמֶנֶת לוֹמַר מֵתָה אֲחוֹתִי שֶׁתִּכָּנֵס לְבֵיתָהּ. וְאֵין הָאִישׁ נֶאֱמָן לוֹמַר מֵתָה אִשְׁתִּי שֶׁיִּשָּׂא אֶת אֲחוֹתָהּ עַד שֶׁיָּעִידוּ שְׁנֵי עֵדִים שֶׁמֵּתָה אֲחוֹתָהּ וְאַחַר כָּךְ תִּכָּנֵס לְבֵיתָהּ. שֶׁלֹּא הֶאֱמִינוּ עֵד אֶחָד אֶלָּא מִשּׁוּם הַתָּרַת עֲגוּנָה כְּמוֹ שֶׁבֵּאַרְנוּ:
כסף משנה
11.
A woman's testimony is accepted with regard to the death of her husband, and she may marry [another man] or perform yibbum [on this basis]. Nevertheless, the testimony of a yevamah is not accepted with regard to the death of her yavam, and it is not considered to be sufficient basis for her to marry another man. The rationale is that the prohibition [against marrying another man when under obligation to a yavam] involves merely a negative commandment.24The punishment - lashes - is lesser, and the woman's children are not considered illegitimate. And therefore the woman may regard it casually.25Having relations with another man while married, by contrast, is a prohibition punishable by execution by the court, her children are considered illegitimate, and it brings severe retribution in the world to come. Because the prohibition against marrying while under obligation to a yavam is less severe, our Sages feared that the woman might lie about the yavam's death and transgress this prohibition.
Similarly, the testimony of a yavam is not accepted with regard to the death of his brother, and it is not considered to be sufficient basis for him to marry his brother's wife. [We fear that] perhaps he desired her.
Similarly, a woman's testimony is not accepted with regard to the death of her sister, and it is not considered to be sufficient basis for her to be permitted to marry her sister's husband.26It is forbidden for a husband to marry his wife's sister while his wife is alive. Our Sages feared that the second sister or the husband may desire to give false testimony, so that they can marry. [By the same token,] a man's testimony that his wife has died is not accepted as sufficient basis for him to marry her sister. In this instance, it is necessary that two witnesses testify that one sister has died before the other sister can enter her household. For the testimony of one witness was accepted only so that license to marry could be granted to a woman who would otherwise be forced to remain unmarried, as we have explained.27I.e., generally, the testimony of two witnesses is required with regard to matters involving marriage and divorce. As mentioned in the concluding passage of Hilchot Gerushin (cited in the notes on Halachah 6), in order to allow a woman to remarry, leniency is granted and we rely on the testimony of one witness. In this instance, however, there is no difficulty in the man's or the woman's remarrying; the only difficulty is that they cannot marry each other. In such an instance, our Sages were not willing to extend leniency.
[There is a slight difficulty when comparing the Rambam's statements here to his statements at the conclusion of Hilchot Gittin. As explained by the Noda BiY'hudah (Even HaEzer, Volume I, Responsa 27 and 33), the Rambam's wording indicates that, according to Scriptural law, with regard to a point of fact that can be verified, it is not necessary for two witnesses to testify, and the statements of one witness are sufficient. The leniency granted by our Sages was to accept the testimony of women, relatives and others whose testimony would not ordinarily be accepted. Here, however, the Rambam is stating (as he does in Hilchot Edut 5:2) that the acceptance of the testimony of a single witness is a leniency of Rabbinic origin. (See the Responsa of the Rivash, Responsum 181.)]

הלכה יב
לְפִיכָךְ הָאִשָּׁה שֶׁהָלְכָה הִיא וּבַעְלָהּ וִיבָמָהּ לִמְדִינַת הַיָּם וּבָאָה וְאָמְרָה מֵת בַּעְלִי וְאַחַר כָּךְ מֵת יְבָמִי. אוֹ שֶׁאָמְרָה מֵת יְבָמִי וְאַחַר כָּךְ מֵת בַּעְלִי הֲרֵי זוֹ אֵינָהּ נֶאֱמֶנֶת. אֲבָל אִם הָלְכָה הִיא וּבַעְלָהּ בִּלְבַד וּבָאָה וְאָמְרָה נִתַּן לִי יָבָם בִּמְדִינַת הַיָּם וָמֵת. בֵּין שֶׁאָמְרָה מֵת יְבָמִי וְאַחַר כָּךְ מֵת בַּעְלִי בֵּין שֶׁאָמְרָה מֵת בַּעְלִי וְאַחַר כָּךְ מֵת הַיָּבָם שֶׁנִּתַּן לִי הֲרֵי זוֹ נֶאֱמֶנֶת שֶׁהַפֶּה שֶׁאָסַר הוּא הַפֶּה שֶׁהִתִּיר:
כסף משנה
12.
Accordingly,28I.e., based on the premise stated in the first clause of the previous halachah. when a woman, her husband and her yavam journey overseas, and she comes and says: "My husband died, and afterwards my yavam died," or "My yavam died, and then my husband died," her word is not accepted.29For the prevailing presumption when she departed was that she would be obligated to perform yibbum if her husband dies.
If, however, she and her husband depart alone, and she returns and says: "A yavam was born for me overseas, but he died," her word is accepted, regardless of whether she says "My yavam died, and then my husband died," or "My husband died, and afterwards the yavam who was born for me died." [The rationale is] the source for the statements [on which basis the woman was] forbidden [states that she is] permitted.30The prevailing presumption when she departed was that she would not be obligated to perform yibbum if her husband dies. It is only because of her own statements that we might suspect that she is required to do so. Therefore, her statements are sufficient to offset this suspicion.

הלכה יג
הָאִשָּׁה שֶׁהָלְכָה הִיא וּבַעְלָהּ וּבְנָהּ לִמְדִינַת הַיָּם וּבָאָה וְאָמְרָה מֵת בַּעְלִי וְאַחַר כָּךְ מֵת בְּנִי נֶאֱמֶנֶת. שֶׁהֲרֵי הָיְתָה בְּחֶזְקַת הֶתֵּר לְזָר בְּעֵת שֶׁהָלְכָה. אָמְרָה מֵת בְּנִי וְאַחַר כָּךְ מֵת בַּעְלִי אֵינָהּ נֶאֱמֶנֶת לְהִתְיַבֵּם וְחוֹשְׁשִׁין לִדְבָרֶיהָ וְחוֹלֶצֶת וְלֹא מִתְיַבֶּמֶת:
כסף משנה
13.
[The following rules apply when] a woman journeys overseas together with her husband and her son: If she returns and says: "My husband died and then my son died," her word is accepted, for at the time she departed the prevailing presumption was that she was permitted to marry another man.
If she says: "My son died, and afterwards my husband died," her word is not accepted as the basis for yibbum.31We suspect that perhaps she is lying in order to marry her brother-in-law. Nevertheless, we grant a certain degree of credence to her words,32I.e., were it not for her statement that her son died first, she would be permitted to marry without any restrictions. Because we give her statements some credence, we require her to perform chalitzah. and she should perform chalitzah, but not yibbum.

הלכה יד
הָלְכָה הִיא וּבַעְלָהּ בִּלְבַד וּבָאָה וְאָמְרָה נִתַּן לִי בֵּן בִּמְדִינַת הַיָּם וָמֵת וְאַחַר כָּךְ מֵת בַּעְלִי נֶאֱמֶנֶת וּמִתְיַבֶּמֶת שֶׁהֲרֵי הָיְתָה בְּחֶזְקַת הֶתֵּר לִיבָמָהּ בְּעֵת שֶׁהָלְכָה. אָמְרָה מֵת בַּעְלִי וְאַחַר כָּךְ מֵת הַבֵּן שֶׁנִּתַּן לִי אֵינָהּ נֶאֱמֶנֶת לִפְטֹר עַצְמָהּ מִן הַיִּבּוּם וּמִן הַחֲלִיצָה וְחוֹשְׁשִׁין לִדְבָרֶיהָ וְחוֹלֶצֶת וְלֹא מִתְיַבֶּמֶת:
כסף משנה
14.
[The following rules apply when] a woman journeys overseas together with her husband alone: If she says, "I "bore a son overseas, he died, and then my husband died," her word is accepted and permitted her to perform yibbum. [The rationale is that] the prevailing presumption when she departed was that she was permitted to perform yibbum.
If she says: "My husband died, and then the son who was born to me died," her statements are not accepted as grounds to free her of the obligation for yibbum or chalitzah. Nevertheless, we grant a certain degree of credence to her words, and she should perform chalitzah, but not yibbum.33I.e., were it not for her statement that her son died after her husband, she would be required to perform yibbum. We do not accept her word to the extent that we allow her to marry without restrictions. Nevertheless, we give her statements some credence and forbid the performance of yibbum, lest in fact there be no obligation to do so.

הלכה טו
בַּמֶּה דְּבָרִים אֲמוּרִים בְּשֶׁהָיְתָה פְּסוּלָה לִכְהֻנָּה מִתְּחִלָּה כְּגוֹן שֶׁהָיְתָה גְּרוּשָׁה אוֹ חֲלָלָה אוֹ שֶׁאָמְרָה בַּמְּעָרָה הָיִינוּ כְּשֶׁמֵּת. אֲבָל אִם אֵין הַדָּבָר כֵּן אֵינָהּ חוֹלֶצֶת. שֶׁמָּא תַּחֲלֹץ וְיָבוֹאוּ עֵדִים וְיָעִידוּ שֶׁהַדָּבָר כְּמוֹ שֶׁאָמְרָה וְהַבַּעַל מֵת תְּחִלָּה וְנִמְצֵאת חֲלִיצָה זוֹ אֵינָהּ כְּלוּם וְתִנָּשֵׂא לְכֹהֵן וְיִרְאֶה הָרוֹאֶה אוֹתָהּ שֶׁחָלְצָה וְנִשֵּׂאת לְכֹהֵן וִידַמֶּה שֶׁהַחֲלוּצָה מֻתֶּרֶת לִכְהֻנָּה וְהוּא אֵינוֹ יוֹדֵעַ בָּעֵדִים שֶׁבָּאוּ. לְפִיכָךְ לֹא תַּחֲלֹץ וְלֹא תִּתְיַבֵּם אֶלָּא תִּשָּׁאֵר בְּחֶזְקַת זְקוּקָה כְּשֶׁיָּצְאָה עַד שֶׁיָּבוֹאוּ עֵדִים:
כסף משנה
15.
When does the above apply? When she was forbidden to the priesthood at the outset - e.g., she had been divorced or a chalalah34A woman who is born from a relationship that is forbidden to a priest, but not to an Israelite. beforehand,35In which case, the chalitzah does not make any difference, for the woman was forbidden to the priesthood regardless. - or she said: "We were [alone] in a cave when he died."36In which case, there is no apprehension of witnesses coming.
If this is not the situation, she may not perform chalitzah, lest she perform chalitzah and then witnesses come and confirm her statements that her husband died first. Thus, it will be evident that this chalitzah was unnecessary, and she [is permitted to] marry a priest. An observer may see that she performs chalitzah and then marries a priest, and he would think that a priest is permitted to marry a woman who performed chalitzah, for he is unaware that witnesses came. Therefore, she should not perform chalitzah, nor should she perform yibbum.37For according to her testimony, she is forbidden to do so. Instead, she should remain under the obligation to perform yibbum as she was when she departed, until witnesses come [and clarify the situation].38Note the discussion of this ruling by the Beit Shmuel 156:17, and the opinion mentioned there that this ruling applies only when there were witnesses who testified that the woman's husband and son both died. Otherwise, the woman's word would be accepted based on the principle of migo - i.e., if she desired to lie, she could have said that her son was still alive.

הלכה טז
וְכֵן אִשָּׁה שֶׁהָלַךְ בַּעְלָהּ וְצָרָתָהּ לִמְדִינַת הַיָּם וּבָאוּ שְׁנַיִם וְאָמְרוּ לָהּ מֵת בַּעְלֵךְ הֲרֵי זוֹ לֹא תַּחֲלֹץ וְלֹא תִּתְיַבֵּם לְעוֹלָם עַד שֶׁיִּוָּדַע אִם יָלְדָה צָרָתָהּ אוֹ לֹא יָלְדָה. וְלָמָּה לֹא תַּחֲלֹץ אַחַר תִּשְׁעָה חֳדָשִׁים מִמִּיתַת הַבַּעַל וְתִהְיֶה מֻתֶּרֶת לְזָר עַל כָּל פָּנִים שֶׁאִם יָלְדָה צָרָתָהּ הֲרֵי נִפְטְרָה זוֹ וְאִם לֹא יָלְדָה הֲרֵי נֶחְלְצָה. גְּזֵרָה שֶׁמָּא יִוָּדַע אַחַר הַחֲלִיצָה שֶׁיָּלְדָה צָרָתָהּ וָלָד שֶׁל קַיָּמָא וְנִמְצֵאת זֹאת שֶׁאֵינָהּ חֲלוּצָה וְתִנָּשֵׂא לְכֹהֵן אַחַר שֶׁנֶּחְלְצָה וְיֹאמַר הָרוֹאֶה שֶׁלֹּא יָדַע בָּעֵדִים שֶׁבָּאוּ שֶׁהַחֲלוּצָה מֻתֶּרֶת לְכֹהֵן וְיָעִיד שֶׁרָאָה אוֹתָהּ נִשֵּׂאת לִכְהֻנָּה עַל פִּי בֵּית דִּין. לְפִיכָךְ אִם הָיְתָה אֲסוּרָה לִכְהֻנָּה מִתְּחִלָּתָהּ הֲרֵי זוֹ חוֹלֶצֶת לְאַחַר תִּשְׁעָה וְתִנָּשֵׂא לְזָר. אֲבָל זוֹ הַצָּרָה שֶׁהָיְתָה עִם בַּעְלָהּ כְּשֶׁמֵּת תַּמְתִּין תִּשְׁעִים יוֹם כִּשְׁאָר הַיְבָמוֹת וְתַחְלֹץ אוֹ תִּתְיַבֵּם וְלֹא תָּחוּשׁ לְצָרָתָהּ שֶׁבַּמְּדִינָה הָאַחֶרֶת הוֹאִיל וְלֹא הָיָה בַּעְלָהּ עִמָּהּ בַּמְּדִינָה:
כסף משנה
16.
Similar laws apply when a woman's husband and a second wife journeyed overseas, and two witnesses came and told [the first wife], "Your husband died." She should not perform chalitzah, nor should she perform yibbum until she knows whether or not her husband's second wife bore a child.39This law applies even if the second wife was not pregnant when she left. Our Sages considered the possibility of the woman becoming pregnant as a reasonable probability.
Why should she not perform chalitzah nine months after the death of her husband, in which case she would be permitted to marry another man from any standpoint? If the other woman bore a child, that frees her of all obligations. And if the other woman did not bear a child, she [should be permitted, because] she performed chalitzah.
[Our Sages prohibited this] as a decree, lest after the woman performed chalitzah, it become known that her husband's other wife bore a viable child. This woman will then not be [bound by the prohibitions of] a woman who performed chalitzah, and she may marry a priest despite performing chalitzah. An observer - who did not know about the witnesses [who informed her of the birth of her husband's child] - might conclude that a priest is permitted to marry a woman who performed chalitzah, and he will testify that a Rabbinic court gave her license to marry a priest. Therefore, if she was forbidden to the priesthood at the outset, she may perform chalitzah nine [months after the death of her husband and] she may marry another man.
The wife who accompanied her husband when he died should wait 90 days, as other yevamot.40See Chapter 1, Halachah 19. Thus, she may perform chalitzah or yibbum. She need not worry about [the possibility of] the wife in the country [giving birth], for that woman was not in the same country as her husband.41We do not consider the possibility that the woman was impregnated by her husband before he departed on the journey and did not realize this until he had already departed.

הלכה יז
הָאִשָּׁה שֶׁמֵּת בַּעְלָהּ וְהָיְתָה לָהּ חָמוֹת בִּמְדִינַת הַיָּם אֵינָהּ חוֹשֶׁשֶׁת שֶׁמָּא יָלְדָה חֲמוֹתָהּ וּכְבָר נִתַּן לָהּ יָבָם בִּמְדִינָה אַחֶרֶת. שֶׁלֹּא גָּזְרוּ בְּדָבָר זֶה. אֶלָּא נַעֲמִיד אוֹתָהּ עַל חֶזְקָתָהּ וַהֲרֵי הִיא מֻתֶּרֶת. וְהוּא הַדִּין לְאִשָּׁה שֶׁמֵּת בַּעְלָהּ וְהָיָה לוֹ בֵּן בִּמְדִינָה אַחֶרֶת הֲרֵי זוֹ מֻתֶּרֶת לְזָר וְאֵין חוֹשְׁשִׁין שֶׁמָּא מֵת הַבֵּן אֶלָּא הַעֲמֵד אוֹתָהּ עַל חֶזְקָתָהּ:
כסף משנה
17.
When a woman's husband dies, and her mother-in- law is living overseas, she need not suspect that her mother-in-law bore a child [before her husband's death], and that there is a yavam for her living in another country. Our Sages did not institute a decree of this nature.42For even if we presume that the mother-in-law will become pregnant, it is more probable that she will miscarry or give birth to a girl (in which cases, the mitzvah of yibbum would not apply) than that she bear a son (Beit Shmuel 157:11). Instead, they allowed the prevailing presumption [regarding her status to continue], and she is permitted to remarry.
The same concept applies when a woman's husband dies, and her son is living overseas - she need not suspect that her son died [before her husband's death]. Instead, we allow the prevailing presumption [regarding her status to continue].

הלכה יח
יָצָאת חֲמוֹתָהּ מְעֻבֶּרֶת הֲרֵי זוֹ חוֹשֶׁשֶׁת וְלֹא תִּנָּשֵׂא לְזָר עַד שֶׁתֵּדַע מֶה הָיָה סוֹף עִבּוּר חֲמוֹתָהּ שֶׁמָּא נוֹלַד לָהּ יָבָם קֹדֶם מִיתַת הַבַּעַל:
כסף משנה
18.
If, however, the mother-in-law departed [overseas] while she was pregnant, the daughter-in-law must bear that fact in mind. She may not marry another man until she knows the fate of her mother-in-law's pregnancy. Perhaps a yavam for her was born before her husband died.

הלכה יט
הָאִשָּׁה שֶׁהָלַךְ בַּעְלָהּ וּבְנָהּ לִמְדִינַת הַיָּם וּבָאוּ וְאָמְרוּ לָהּ מֵת בַּעְלֵךְ וְאַחַר כָּךְ מֵת בְּנֵךְ וְנִשֵּׂאת וְאַחַר כָּךְ נוֹדַע שֶׁחִלּוּף הָיוּ הַדְּבָרִים תֵּצֵא וְהַוָּלָד כָּשֵׁר. אָמְרוּ לָהּ מֵת בְּנֵךְ וְאַחַר כָּךְ מֵת בַּעְלֵךְ וְנִתְיַבְּמָה וְאַחַר כָּךְ נוֹדַע שֶׁהָיוּ הַדְּבָרִים חִלּוּף תֵּצֵא וְהַוָּלָד שֶׁנּוֹלַד לִפְנֵי הַשְּׁמוּעָה אוֹ לְאַחַר הַשְּׁמוּעָה מַמְזֵר:
כסף משנה
19.
[The following laws apply when] a woman's husband and son went overseas, and [witnesses] came and told her: "Your husband died and then your son died." If she married,43For according to the testimony she was given, her husband was survived by a son, and the mitzvot of yibbum and chalitzah do not apply. and later discovered that they died in the opposite order,44In which case her husband would have died without children, and she would be obligated to perform yibbum or chalitzah. she must leave her second husband.45See Chapter 2, Halachah 18. None of the children she bore him is considered illegitimate, however.46For it is only when a child is born from forbidden relations that are punishable by karet that this status is incurred (Yevamot 49a).
[More stringent rules apply if she was told: "Your son died and then your husband died," and on that basis she performed the rite of yibbum and then discovered that they died in the opposite order. She is obligated to leave her second husband. All the children she bore him - both those born before she received this notice and those born afterwards - are considered illegitimate.47For her relations with her brother-in-law were prohibited by a transgression punishable by karet. The fact that she violated this prohibition unwittingly does not change the status of her children.

נשים הלכות יבום וחליצה פרק ג
Nashim Yibum and Chalitzah Chapter 3