Halacha

הלכה א
הַקְּרוֹבִים פְּסוּלִים לְעֵדוּת מִן הַתּוֹרָה שֶׁנֶּאֱמַר (דברים כד טז) "לֹא יוּמְתוּ אָבוֹת עַל בָּנִים". מִפִּי הַשְּׁמוּעָה לָמְדוּ שֶׁבִּכְלַל לָאו זֶה שֶׁלֹּא יוּמְתוּ אָבוֹת עַל פִּי בָּנִים וְלֹא בָּנִים עַל פִּי אָבוֹת. וְהוּא הַדִּין לִשְׁאָר קְרוֹבִים. אֵין פְּסוּלִין מִדִּין תּוֹרָה אֶלָּא קְרוֹבִים מִמִּשְׁפַּחַת אָב בִּלְבַד. וְהֵם הָאָב עִם הַבֵּן וְעִם בֶּן הַבֵּן. וְהָאַחִין מִן הָאָב זֶה עִם זֶה וּבְנֵיהֶן זֶה עִם זֶה וְאֵין צָרִיךְ לוֹמַר הַדּוֹדִים עִם בֶּן אָחִיו. אֲבָל שְׁאָר הַקְּרוֹבִים מִן הָאֵם אוֹ מִדֶּרֶךְ הָאִישׁוּת כֻּלָּן פְּסוּלִין מִדִּבְרֵיהֶם:
כסף משנה
1.
Relatives are disqualified as witnesses according to Scriptural Law, as implied by Deuteronomy 24:16: "Fathers shall not die because of sons." According to the Oral Tradition, the verse is interpreted as meaning that included in this prohibition is that fathers should not die because of the testimony of sons, nor should sons die because of the testimony of fathers. Similar laws apply with regard to other relatives.
According to Scriptural Law, only paternal relatives are disqualified - i.e., a father with his sons and grandsons, paternal brothers with each other, and the sons of paternal brothers with each other. Needless to say the uncles may not testify with their brother's sons. Maternal relatives or people related by marriage are disqualified only by Rabbinic decree.

הלכה ב
הַגֵּרִים אֵינָן בְּדִין הַקְּרוֹבִים. אֲפִלּוּ שְׁנֵי אַחִים תְּאוֹמִים שֶׁנִּתְגַּיְּרוּ מְעִידִין זֶה לָזֶה. שֶׁהַגֵּר שֶׁנִּתְגַּיֵּר כְּקָטָן שֶׁנּוֹלַד הוּא חָשׁוּב:
כסף משנה
2.
Converts are not considered as relatives. Even two twin brothers who convert may testify on each others behalf. For a convert is considered as a newborn child.

הלכה ג
הָאַחִים זֶה עִם זֶה בֵּין מִן הָאֵם בֵּין מִן הָאָב הֲרֵי הֵן רִאשׁוֹן בְּרִאשׁוֹן. וּבְנֵיהֶם זֶה עִם זֶה שֵׁנִי בְּשֵׁנִי. וּבְנֵי בְּנֵיהֶם זֶה עִם זֶה שְׁלִישִׁי בִּשְׁלִישִׁי:
כסף משנה
3.
Brothers - whether maternal brothers or paternal - are considered as one degree removed. Their sons are considered as two degrees removed. And their grandsons are three degrees removed.

הלכה ד
וּלְעוֹלָם שְׁלִישִׁי בְּרִאשׁוֹן כָּשֵׁר וְאֵין צָרִיךְ לוֹמַר שְׁלִישִׁי בְּשֵׁנִי. אֲבָל שֵׁנִי בְּשֵׁנִי וְאֵין צָרִיךְ לוֹמַר שֵׁנִי בְּרִאשׁוֹן שְׁנֵיהֶם פְּסוּלִים:
כסף משנה
4.
A person who is three degrees removed may testify on behalf of one who is one degree removed. Needless to say, one who is three degrees removed may testify on behalf of one who is two degrees removed. But two who are both two degrees removed, and needless to say, one who is two degrees removed and one who is one degree removed are both disqualified from testifying.

הלכה ה
הָאָב עִם בְּנוֹ כְּרִאשׁוֹן בְּרִאשׁוֹן הוּא. לְפִיכָךְ הָאָב עִם בֶּן בְּנוֹ פָּסוּל. וְעִם בֶּן בֶּן בְּנוֹ שֶׁהוּא רְבִיעִי מִמֶּנּוּ כָּשֵׁר מִפְּנֵי שֶׁהוּא שְׁלִישִׁי בְּרִאשׁוֹן. וְכֵן הַדֶּרֶךְ בִּנְקֵבוֹת. כֵּיצַד. שְׁתֵּי אֲחָיוֹת אוֹ אָח וַאֲחוֹתוֹ בֵּין מִן הָאָב בֵּין מִן הָאֵם הֲרֵי הֵם רִאשׁוֹן בְּרִאשׁוֹן. בְּנֵיהֶם בֵּין זְכָרִים בֵּין נְקֵבוֹת שֵׁנִי בְּשֵׁנִי. בְּנֵי בְּנֵיהֶם אוֹ בְּנוֹת בְּנוֹתֵיהֶן שְׁלִישִׁי בִּשְׁלִישִׁי. כְּדֶרֶךְ שֶׁאַתָּה מוֹנֶה בִּזְכָרִים רִאשׁוֹן שֵׁנִי וּשְׁלִישִׁי כָּךְ אַתָּה מוֹנֶה בִּנְקֵבוֹת:
כסף משנה
5.
A father and his son are considered as one degree removed. Therefore a father is disqualified from testifying with his grandson. With his great-grandson, i.e., the fourth generation, he is acceptable, for he is of the first degree and the great-grandson, three degrees, removed. Similar laws apply with regard to women relatives.
What is implied? Two sisters or a brother and a sister - whether paternally or maternally related - are considered as one degree removed. Their children whether male or female are considered as two degrees removed and their grandchildren - including the sons of their sons and the daughters of their daughters - are considered as three degrees removed. Just as we count the degrees descendants are removed for males - one degree, two degrees, and three degrees - so, too, we count the degrees for females.

הלכה ו
כָּל אִשָּׁה שֶׁאַתָּה פָּסוּל לָהּ כָּךְ אַתָּה פָּסוּל לְבַעְלָהּ שֶׁהַבַּעַל כְּאִשְׁתּוֹ. וְכָל בַּעַל שֶׁאַתָּה פָּסוּל לוֹ כָּךְ אַתָּה פָּסוּל לְאִשְׁתּוֹ שֶׁהָאִשָּׁה כְּבַעְלָהּ:
כסף משנה
6.
Whenever a person is disqualified from testifying on behalf of a woman, he is also disqualified from testifying on behalf of her husband, for a husband is considered like his wife. Conversely, whenever a person is disqualified from testifying on behalf of a man, he is also disqualified from testifying on behalf of his wife, for a wife is considered like her husband.

הלכה ז
כָּל שְׁתֵּי נָשִׁים שֶׁהֵן זוֹ עִם זוֹ שֵׁנִי בְּשֵׁנִי בַּעְלֵיהֶן מְעִידִין זֶה לָזֶה. אֲבָל אִם הָיָה רִאשׁוֹן בְּרִאשׁוֹן כְּגוֹן שֶׁלָּקַח זֶה אִשָּׁה וְזֶה בִּתָּהּ אֵין מְעִידִין זֶה לָזֶה:
כסף משנה
7.
Whenever two women are removed by second degrees, their husbands may testify on behalf of each other. If, however, the women are one degree removed, e.g., a man married a woman, and a colleague married her daughter, the two men may not testify on each other's behalf.

הלכה ח
וְכֵן בַּעְלֵי אֲחָיוֹת פְּסוּלִין זֶה לָזֶה וַהֲרֵי הֵן רִאשׁוֹן בְּרִאשׁוֹן:
כסף משנה
8.
Similarly, the husbands of two sisters are disqualified with regard to each other and are considered as one degree removed.

הלכה ט
וְכֵן לֹא יָעִיד לְבֶן אֲחוֹת אִשְׁתּוֹ וְלֹא לְבַעַל בַּת אֲחוֹת אִשְׁתּוֹ. אֲבָל מֵעִיד הוּא לְבֶן בַּעַל אֲחוֹת אִשְׁתּוֹ שֶׁיֵּשׁ לוֹ מֵאִשָּׁה אַחֶרֶת:
כסף משנה
9.
Similarly, a person should not testify on behalf of the son of his wife's sister, nor on behalf of the husband of the daughter of his wife's sister. He may, however, testify on behalf of the son of the husband of his wife's sister who was born to that person from another wife.

הלכה י
כָּל אִישׁ שֶׁאֵין אַתָּה מֵעִיד לוֹ מִפְּנֵי שֶׁהוּא בַּעַל קְרוֹבָתְךָ הֲרֵי אַתָּה מֵעִיד לִשְׁאָר קְרוֹבָיו כְּגוֹן בְּנוֹ וְאָחִיו. וְכֵן כָּל אִשָּׁה שֶׁאֵין אַתָּה מֵעִיד לָהּ מִפְּנֵי שֶׁהִיא אֵשֶׁת קְרוֹבְךָ הֲרֵי אַתָּה מֵעִיד לִשְׁאָר קְרוֹבֶיהָ:
כסף משנה
10.
Whenever a witness is disqualified from testifying on behalf of a colleague because he is married to the witness' relative, the witness may testify on behalf of the other relatives of that colleague, e.g., his son and his brother. Similarly, whenever a witness is disqualified from testifying on behalf of a woman because she is married to the witness' relative, the witness may testify on behalf of her other relatives, e.g., her son and her brother.

הלכה יא
אֲבִי כַּלָּה וַאֲבִי חָתָן מְעִידִים זֶה לָזֶה:
כסף משנה
11.
The father of a bride and the father of a groom may testify on behalf of each other.

הלכה יב
אֲחֵי הָאָח מִן הָאֵם מְעִידִין זֶה לָזֶה שֶׁהֲרֵי אֵין בֵּינֵיהֶן קֻרְבָה כְּלָל. כֵּיצַד. רָחֵל שֶׁנִּשֵּׂאת לְיוֹסֵף וְיָלְדָה מִמֶּנָּה רְאוּבֵן. וְהָיָה לְיוֹסֵף בֵּן מֵאִשָּׁה אַחֶרֶת וְהוּא מְנַשֶּׁה. וּמֵת יוֹסֵף וְנִשֵּׂאת לְשִׁמְעוֹן וְיָלְדָה מִמֶּנּוּ יְהוּדָה. הֲרֵי מְנַשֶּׁה וִיהוּדָה מְעִידִין זֶה לָזֶה:
כסף משנה
12.
A person's paternal brother may testify on behalf of that person's maternal brother and the maternal brother may testify on behalf of the paternal brother, for they have no connection whatsoever.
What is implied? Rachel married Joseph and bore him a son Reuven. Joseph had a son, Menashe, from another wife. Joseph died and Rachel married Shimon and bore him Judah. Menashe and Judah may testify on each other's behalf.

הלכה יג
הָאִישׁ עִם אִשְׁתּוֹ רִאשׁוֹן בְּרִאשׁוֹן. לְפִיכָךְ אֵינוֹ מֵעִיד לֹא לִבְנָהּ. וְלֹא לְאֵשֶׁת בְּנָהּ. וְלֹא לְבִתָּהּ. וְלֹא לְבַעַל בִּתָּהּ. וְלֹא לְאָבִיהָ. וְלֹא לְאִמָּהּ. וְלֹא לְבַעַל אִמָּהּ. וְלֹא לְאֵשֶׁת אָבִיהָ:
כסף משנה
13.
A man and his wife are considered as being removed by one degree. Hence a husband may not testify on behalf of his wife's son, the wife of his wife's son, his wife's daughter, the husband of his wife's daughter, his wife's father, his wife's mother, the husband of his wife's mother, nor the wife of his wife's father.

הלכה יד
אִשְׁתּוֹ אֲרוּסָה אַף עַל פִּי שֶׁלֹּא נִגְמְרָה הָאִישׁוּת הֲרֵי הִיא כִּנְשׂוּאָה לְעִנְיַן עֵדוּת. [בַּמֶּה דְּבָרִים אֲמוּרִים] בַּאֲרוּסָתוֹ עַצְמָהּ שֶׁאֵינוֹ מֵעִיד לָהּ. אֲבָל אִם הֵעִיד לִקְרוֹבֵי אֲרוּסָתוֹ כְּגוֹן בַּעַל אֲחוֹתָהּ אוֹ בְּנָהּ וּבִתָּהּ וְכַיּוֹצֵא בָּהֶן אֵין פּוֹסְלִין אוֹתָהּ עַד שֶׁיִּשָּׂאֶנָּה:
כסף משנה
14.
When a person has consecrated a woman, although the marriage bond has not been consummated, with regard to testimony, it is as if she is fully married.
To whom does the above apply? To the woman he consecrated herself; he may not testify on her behalf. If, however, he testifies on behalf of the relatives of the woman he consecrated, e.g., her sister's husband, her son or daughter, or the like, we do not disqualify him until he marries her.

הלכה טו
זֶה שֶׁפָּסְלָה תּוֹרָה עֵדוּת הַקְּרוֹבִים לֹא מִפְּנֵי שֶׁהֵן בְּחֶזְקַת אוֹהֲבִין זֶה אֶת זֶה. שֶׁהֲרֵי אֵינוֹ מֵעִיד לוֹ לֹא לְטוֹבָתוֹ וְלֹא לְרָעָתוֹ. אֶלָּא גְּזֵרַת הַכָּתוּב הוּא. לְפִיכָךְ הָאוֹהֵב וְהַשּׂוֹנֵא כָּשֵׁר לְעֵדוּת אַף עַל פִּי שֶׁהוּא פָּסוּל לְדַיָּנוּת. שֶׁלֹּא גָּזְרָה תּוֹרָה אֶלָּא עַל הַקְּרוֹבִים:
כסף משנה
15.
The Torah did not disqualify the testimony of relatives because we assume that they love each other, for a relative may not testify neither on his relative's behalf or against his interests. Instead, this is a Scriptural decree.
For this reason people who love each other or who hate each other are acceptable as witnesses even though they are not acceptable as judges. For the Scriptural decree disqualifies only relatives as witnesses.

שופטים הלכות עדות פרק יג
Shoftim Edus Chapter 13