הלכה על סנהדרין 54:2

That a relative not testify, one about the other: That we have been commanded not to accept the testimony of some relatives about some relatives. And about this is it stated (Deuteronomy 24:16), "Parents shall not be put to death for children, nor children be put to death for parents" - and concerning this came the traditional understanding (Sanhedrin 27b) [that it means that] fathers should not be put to death from the testimony of the sons, and sons should not be put to death from the testimony of the fathers. And the same is true of financial cases - that we do not believe some relatives about some relatives. However it is mentioned regarding laws [involving] death sentences by way of hyperbole - that we should not say, since this is [causing] the loss of life, we should not suspect the relative about him, but rather do like his testimony; since his testimony is to lose the life of his relative. And even though the verse only mentioned the fathers and the sons, the same is true of several of the other relatives. But it mentioned sons and fathers by way of an example; saying that [even though] one loves the other very much, and [yet the Torah] stated that also these are not believed that one should condemn the other and - there is no need to say - exonerate [him]. And the same is true of some of the [other] relatives, as the tradition about them has come to us - and we shall mention them in the laws of the commandment, with God's help.