What do I do?

So my brother-in-law, abuser, rapist, molostor, whatever you want to call him. He has a son. Who is very very wild. He likes to get into anything. And it is just very crazy with him. Well when I usually see my nephew he usually has scratches because he just falls a lot. Well today I saw my nephew. And he had like 5 bruises on his arm. And my other sister (not the mom) asked my abuser what happened to him and he said I don't know know. There are too many bruises to count. And then my sister asked my other sister (mom of my nephew) what happened. And she just said I don't know. And just didn't care. I am worried my nephew is in danger. What should I do?? I need advice!!!