The Gemara in Makos (6a) states that if two brothers saw the incident, we ask them if they came merely to be onlookers or if they came to testify. If they merely came to watch, their testimony is valid. The Rambam (Hilchos Edus 5:4), writes that when they are close relatives, their testimony is invalid only when they intended to give testimony. If they did not intend to testify, then the testimony of the one who does testify (such as with another witness) is valid because otherwise two brothers who saw the same incident would always invalidate any possible testimony.

Kol Tuv,

Dovid Bloom