Relatives' Testimony

"Fathers shall not be put to death on account of their children; neither shall children be put to death on account of their fathers"Deuteronomy 24:16.

A judge may not accept testimony from a litigant's close relative—whether the testimony is in support of the litigant or against him.

This law applies to all cases – capital and monetary – and all close relatives. The verse refers specifically to a capital case and the testimony of a father to demonstrate the extent of this mitzvah. For the Torah even forbids accepting the testimony of a father, who presumably loves his child dearly, against his child (or vice versa) in a capital case—though we'd presume that such testimony is eminently credible.

This mitzvah is a supra-rational decree from Above.