Parshas Ki Setzei
“Fathers will not be put to death because of their sons and sons will not be put to death because of their fathers!” (Devarim 24:16)
It is a negative commandment that we may not accept the testimony of relatives about each other, as it says “Fathers will not be put to death because of their sons and sons will not be put to death because of their fathers!”. Chazal received by transmission that we may extrapolate from this verse that fathers may not be put to death because of the testimony of their own sons and sons may not be put to death because of the testimony of their own fathers. The same applies in monetary matters in that we may not trust the testimony of one who gives evidence about his own relative. Even though the verse talks explicitly only about fathers and sons, the same applies to certain other relatives who may not testify about each other even on the side of the accuser, and obviously are not believed to absolve them.
Amongst the roots of this commandment are that since all of the affairs of people are dependent on other people’s testimony, Hashem wanted to distance from him that justice should only be carried out between people with strong truthful testimony which is free from any doubt. In order to strengthen this point He distanced all of the testimony of relatives even to accuse their relative in case it would spread to the extent that we would accept it even for innocence. The idea of this is part of the perfect ways of the Torah, in that we should always distance ourselves from possible stumbling-blocks and other things which are likely to cause damage to people. Another achievement of this is that that since relatives live together and go to bed and get up at the same time etc., it is impossible to save them from having occasional quarrels and if we would always believe their testimony about each other, sometimes in their constant anger with each other their anger would rise to such a degree that they will come before a judge and condemn their relative to death, and when the anger subsides the witness will regret having testified because he is worried about his relative and his deeds. For this reason, the Torah says that we should not accept his testimony in the first place and all of the ways of the Torah are straight.
Amongst the laws of this commandment are that anyone who is invalidated from testifying about a woman (because they are related to her), is automatically invalidated from testifying about her husband, as the husband and wife are like one body for these laws. In the same way, anyone who is invalidated from testifying about a man (because they are related to him), is automatically invalidated from testifying about his wife, as the husband and wife are like one body for these laws.
This commandment applies even today regarding monetary judgments which apply today, in all places to men on whom it is incumbent to accept testimony.