There is an important rule relevant to the Halacha of "Kerobim," the law disqualifying relatives from serving as co-witnesses or testifying about one another. The rule is that "Ba’al Ke’ishto Ve’isha Ke’ba’alah" – a husband is considered the same as his wife, and a wife is considered the same as her husband. Meaning, when we determine the nature of a familial relationship between two prospective witnesses, their wives’ relatives are considered their own relatives. Practically speaking, this means that two men could be disqualified from serving as witnesses together or testifying about one another even if they are not blood relatives, and are related only through marriage.
For example, as the Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908) discusses in Hoshen Mishpat (33:4), a person cannot testify for or against his relative’s husband – such as his sister’s husband, or his first cousin’s husband. Even though the two men are related only through marriage, and are not blood relatives, the testimony is disqualified because a husband is considered a direct relative of his wife’s relatives. By the same token, one would be disqualified from testifying about his relative’s wife.
A more complicated application of this rule is a case of two men who are married to sisters. Each husband assumes the same status as his wife, and therefore these two men are regarded as "brothers" with respect to testimony. As such, they would not be allowed to serve as witnesses together, or to testify about one another, even though they are related only through their wives. This would be a case of "Rishon Be’rishon" – testifying with or about a direct family member – since the two men are considered "brothers."
Summary: Just as one cannot testify with or about a relative, one also cannot testify with or about a relative’s husband, or about a relative’s wife. Thus, for example, one cannot serve as a co-witness with his sister’s husband or his cousin’s husband, and may not testify for or against his cousin’s wife or his brother’s wife. Likewise, two men who are married to sisters cannot testify with each other or about one other.
