Harresment. Domestic violence

Dear sir.. i have my elder sister married to a family in chennai.. we have no parents.. my mom passed away a year ago. She was married in 2011. Since then she has been facing harresment. Mental torture. Her husband hits her. Earlier my mom used to go and talk to them and settle this issue. Because no mother would want her daughter's life to get ruined. My brother-in-laws parents are always biased. They never care about their grandchild which is a girl.. my sister has a newborn boy now who's 1. And a girl who's 7. Today my sister's inlaws shouted at the girl kid because she was fighting with a kid who is brother-in-laws sister's son. My sister just said that it was the boy who started the fight and that's it. My brother-in-law hit my sister so hard that her ear started to pain and she pissed her pants. This is mental torture and harresment. This has been the case since a long time but my sister always kept her calm. We don't have anyone to tell. We want to complaint but what will be the future of the kids. We are scared. I'm scared thinking how long this will go.. and what if my sister takes any bad decesion of taking her Life. Please help us