The Shulhan Aruch (Hoshen Mishpat 33) rules that a person cannot serve as a witness if he is a family member of one of the litigants, or of another witness. Similarly, in the case of a wedding, one cannot serve as a witness for the Kiddushin if he is a family member of either the bride or the groom, or of the other witness. Depending on the precise familial relationship, the witness might be disqualified Min Ha’Torah, meaning, on the level of Torah law (as opposed to Rabbinic enactment).
What is the reason for this disqualification?
Instinctively, we might assume that a family member is disqualified from testifying because he will naturally come with a favorable bias, which might lead him to falsely testify on behalf of his kin. In truth, however, the concern of bias cannot be the basis of this Halacha. The Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908), in his introduction to the laws of testimony (listen to audio for precise citation), gives three compelling reasons why we cannot attribute this Halacha to the concern of bias. Firstly, the Gemara states explicitly that one may not even testify against his family member. In such a case, of course, there is no concern of favorable bias – as his testimony is to his family member’s detriment – and yet he is nevertheless disqualified from testifying. Secondly, as mentioned, this disqualification applies even if the witnesses are not related to a litigant, but are related to each other. Their relationship to each other does not give them reason to be favorably disposed to either litigant, and thus their disqualification clearly has nothing to do with the concern of a biased predisposition. Finally, and most compellingly, the Gemara comments that even if Moshe and Aharon would come together to give testimony, their testimony would not be accepted because they are brothers. Quite obviously, nobody would ever suspect Sadikim on the stature of Moshe and Aharon to testify falsely, and yet they would nevertheless be disqualified because of their familial relationship.
Necessarily, then, this Halacha constitutes a "Gezerat Ha’katub," a decree of the Torah whose rationale eludes our comprehension. The Almighty, in His infinite wisdom, determined that a person may not testify if he is related to a litigant or to the other witness, for whatever reason.
Summary: Halacha disqualifies a person from serving as a witness – either in court or in other contexts, such as for Kiddushin at a wedding – if he is a family member of one of the litigants – or of the bride or groom, in the case of Kiddushin – or if he is a family member of the other witness.
