Negative Mitzvah 287
It is forbidden to accept testimony given by a relative of either of the litigants
Deuteronomy 24:16 "Fathers shall not be put to death for (the testimony of) children; neither shall children shall not be put to death for (the testimony of) fathers"
A judge is forbidden to accept testimony of people that are relations of the person on trial.
This Negative Mitzvah applies at all times, whether the relative's testimony can help prove the defendant guilty or innocent.