There was a story that R’ Akiva stopped 40 groups of witnesses from going to Bais Din in Lud on Shabbos. R’ Gamlial sent a message to him that if he stopped these witnesses now, the next time they might not attempt to come.

Rosh Hashana 1:7

Even a father and a son should come to Bais Din to say witness, even though they are relatives who cannot say witness. The reason is that maybe one will become posul [invalid] and the other could be accepted with a different witness.

R’ Shimon holds relatives are kosher for witness regarding kidush hachodesh.