Q: Which relatives are disqualified from serving as witnesses in beis din?
A: The Torah disqualifies witnesses who are relatives of one of the litigants, one of the Dayanim, or of each other — whether to exempt or obligate (C.M. 33:10, 17).
As a rule of thumb, first-degree relatives (e.g., parent–child or siblings) and second-degree relatives (e.g., grandparent–grandchild, uncle–nephew, or first cousins) are disqualified; third-degree relatives are allowed (e.g., second cousins or even first cousins once removed). There is a dispute regarding great-grandparents–grandchildren and great-uncles–nephews, and they should not testify (C.M. 33:2).
Husband and wife are generally considered the same. Thus, a witness disqualified as a relative also cannot testify for the relative’s spouse. However, the unrelated spouses of two first cousins can testify (C.M. 33:3-4).
Mechutanim (a pair of in-laws) are allowed to testify for/with each other, if they don’t have a vested interest in the case (C.M. 33:6).