Sister's husband physically abuses her what can be done

my sister got married in 2007 when she 15. But he already liked a girl and on his mother's force he married our sister.His brother married the same girl whom he liked. By 2010 she had two kids. Then he himself told that he meanwhile developed an illegal relationship with his brothers wife to save himself before that lady reveals and then our elders discussed on this and sent my sister and her husband to live in my mother's house. But he tortured her by beating abusing so on even serially also he harassed her in his brothers wife's memories. He started to say even to my mother that he will again go to his brothers wife. But even his in laws also wanted to hide all these and so started to say everyone that our sister is torturing him by doubting.he left her 7 times and for the last time on 25th January 2016. But now he sent a legal notice saying he's very good person but my sister is doubting him. And my mother is not sending her. what should we do now. we don't have any support.