I am curious. I want to know your opinion on that. I myself was a battered-wife. I had a lot of relatives and ex-relatives who didn't help me while I was being beaten up. Some are even on Geni.
Ignore that stupid relative and help the abused-relative of course! In this case that "blood is thicker than water" rule does not apply.
I would listen to the victim. It is easy to help when the victim sends an SOS. If I am physically close by when the beating happens I would intervene and call the police.