Husband in relationship with other girl, harassment, abuse

Respected sir/madam, Below is the background for my sister's marriage and concern. My sister got married 10 years back to our cousin, it was love marriage however 2 years before the marriage I always saw my sister crying. The status of my brother in law wasn't good therefore my mother decided to let him do the business from our shop (the shop was given to him for business before marriage, he was running an electronic shop at that time now he runs mobile business from same shop). After 2 years of marriage they shifted to our home since then he is living with us. He didn't provide single penny to my sister for anything all expenses are beard by us, before marriage we keep hearing his activities of interest in other girls and even just after marriage he went to hill station with our cousin sisters. We often has provided money to him for his struggle in business and home loan. He always keeps fighting/abusing my sister and doesn't take care of her, during her second pregnancy she used to go to doctor with my mother and even one day she was very much in pain and he was willing to take her to doctor and when my mother requested that she won't be able to take her to doctor then he took her to doctor. His mother and sister says that this marriage is done by their son/brother and we can't help you and have clearly told my mother and sister that don't you ever come to our house. He doesn't let my sister go to any function (marriage, birthdays etc) also he doesn't want my sister to go out so that he won't be caught. Though the shop is ours but he has insulted my sister so much that she does't have enough courage to go there to check if he is doing anything suspicous. My sister is going through mental trauma almost daily, the relation between them is like he is having breakfast in morning and leaving for shop and comes in night and have dinner and they don't speak and if she tries to talk and resolve any conflict that results in insult for her as he start blaming/abusing/fighting with her. when my sister says to him if you can't take care of family and don't want to continue in relations then let me know then his replies are it' s your parents who has given you support and have spoiled my life, though the fact is that before marriage he clevery started creating situations wherein my sister spoke with my mother and he got the shop and financial support to start his business and some money to build his new home and this continuous till now we are supporting my sister for everything (clothing, meal, medical etc). In begging he said he will pay for the shop which my sister communicated to us now he is having 'what do think by giving that shop have you bought me and nobody can take that shop back from me', another example of his smartness is that till 7 years of marriage he never said anything about dowry and recently he said to my sister what your parents have given to me in marriage now I need 10 lacks. He has insulted my mother and father on many occassions and says to my sister he will do something so, so that my parents and both brothers will go to jail and will be in position to sell both houses. What I know is he was once caught by police and currently a case of fraud is under proceeding against him, his background is that he was an accomplise of goon 'Akbar' who was murded in rivarly, he also joined politics as 'Mahanager Adhyaksh' for either Congress or SP in past but now he is no longer active in politics but he do know a lot of people in bareilly and few police inspector which whom he is very close. There are many more things which can be better explained by my sister who lives with him at my hometown in bareilly, I and she is looking for legal advise for how to proceed in this situation. The people who know us says that he married my sister just for money.