What should I do about my sister’s husband's cruelty and dowry issues?

Hi, my sister’s husband doesn’t care for my sister or his newborn baby. During her pregnancy, he neither supported her financially nor emotionally, so she had to stay with my parents. Even now, he doesn’t support her and care for his child. He mentally disturbs my sister. I suspect he has an extramarital affair, as he has done character assassination of my sister. Although she never communicated with her ex-boyfriend after marriage, he used to talk to my sister’s ex-boyfriend’s wife and visit his girlfriend’s house. His father also encourages his bad behavior, as he himself keeps a woman after his wife’s death, and none of this was informed to us until now. They also took dowry from us. What should I do? My parents and sister are very stressed, and my father and sister are carrying all the expenses of the baby.