Im trying to help but i feel useless?

my brother is in a relationship with this girl that is cheating on him and liein to him, they have a son who is 2 and he is belted and starved and always filthy. my brother works long hours and when he is home he feeds, cleans and looks after his son but he is hardley home. i have asked to have my nephew on saturdays and wednesdays and my brother is all for it but she wont let me. she had an abortion a year ago and it wasnt my brothers baby and he knows that but he is scared that if he leaves her she is going to take his son away from him. i want to help my brother and nephew but she is pushin me out of there lifes. what can i do? how can i stop the abuse towards both my brother and nephew? i feel useless!