The Mishna (27b) teaches that people who are closely related – by blood or by marriage – will be disqualified from testifying about one another. Thus a person cannot testify for or against his brother or his sister’s husband, similarly he cannot be a witness regarding a case involving his uncle, whether it is his mother’s brother or his father’s brother, or, for that matter his mother’s sister’s husband or his father’s sister’s husband.
Why are relatives limited in this way?
The Gemara quotes 24:16 as a pasuk that can be interpreted to serve as the source for this law. It appears clear, however, that this is considered a gezeirat ha-katuv – a biblical rule that is not related to a suspicion that the relative may lie on behalf of his brother or his uncle. Even Moshe and Aharon, who we would not suspect of lying, cannot testify about each other.
The Gemara on today’s daf asks about the status of an engaged couple (ishto arusah – “a man’s wife to whom he is engaged” – is already a situation of partial marriage, and is a much closer relationship than what an engagement is today). Rabba bar bar Ḥana believes that a man can testify about ishto arusah. Ravina differentiates between testifying on her behalf or to her detriment. The Gemara concludes that he cannot testify about her at all, explaining that their relationship is one of ikruvei da’ata – that their engagement leads to a sense of affinity, a close emotional relationship, which precludes such testimony.
This explanation appears to suggest that the disqualification from testimony stems from their emotional closeness rather than from the relationship itself. Some suggest that the case of ishto arusah is unique in that the emotional closeness attests to the relationship between the engaged couple, but does not extend to other relatives by marriage (e.g. he could testify about his brother-in-law in such a case).