The 287th prohibition is that a judge is forbidden from accepting the testimony of relatives — whether for each other or against each other.

The source of this prohibition is G‑d's statement1 (exalted be He), "Fathers shall not die through their sons, and sons shall not die through their fathers."

The interpretation of this verse is given in the Sifri,2 "Fathers shall not die through the testimony of their sons, and sons shall not die through the testimony of their fathers."

The same law applies to financial cases, but it was said in a capital case to take the most extreme case. One should not reason, "Since this involves the death penalty, the relative's testimony cannot be doubted, and we shall therefore act in accordance with his words. Since his testimony is causing the death of a relative, there is no doubt." To counter this claim the example is brought of the relatives who have the strongest and greatest love for each other — a father for his son and a son for his father. And even when a father testifies against his son, and even if it is to have him executed, we still cannot accept it. This is a decree of the Torah which has no explanation whatsoever. You should understand this well.

The details of this mitzvah are explained in the third chapter3 of tractate Sanhedrin.