Rambam - Sefer HaMitzvos: Negative Mitzvah 287: Negative Mitzvah 287

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.

In life, we almost never wait for 100% guarantees. We trust that the dentist is a dentist, the taxi driver is a taxi driver, and so on -- and put our lives in their hands -- on flimsy tacit evidence. Yet, when it comes to a simple good deed, people demand 100% proof that this is really what G-d wants them to do!

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We are not waiting for some great revelation from above to save us from our incompetence as guardians of this world and put everything in order. Rather, we are waiting to see the sun rise over everything we have done, to see the fruits of our labors blossom in an eternal spring.
A New Age comes upon the world as a spring rain upon a plowed and seeded field. Plow and sow now, while there is still time.

From: Bringing Heaven Down to Earth by Tzvi Freeman - tzvif@aol.com