Sister's marital life at risk please help

Namaste. My sister got married in 2007. She has two kids. She was 15 at the time of her wedding . Her husband has an illegal affair with his own brothers wife and harrowing my sister from 9 years. Till now he left her at my moms home for 7 times saying your sister is good but I don't want her. When we go for elders to deal this he acts as if he is affectionate to her and when they are alone he beats her badly to say that she don't want him. She tried to suicide 2 times.now at last he left her on 26 Jan 2016. Now he sent a notice that he is very affectionate to his wife and she doubts him and tortures him. Now he says he want his family but my mother is not sending her. What should we do now. We don't have any support. Even psychiatrist also told that he is sadistic and intentionally harassing her. All our relatives say that he is a MAN so u have to tolerate. But she is so scared that her sons also become like their father if he repeats all these. So she is not ready to go with him this time. She feels that let him enjoy how ever he want one day or the other he will realise and come . If his behaviour changes only then I will accept him. We are three Sisters no father no brothers. So he is saying who are there to question me . I will be like this only. He acts like very good outside and tortures her inside. Please help us...