Husband harassing wife

Dear sir/madam I request you to read completely with patience and show me a way to solve this problem. My sister got married in 2011. By now they had 2 year old son . since their marriage her mother in law used to defame her in front of her husband. He just believes his mother. This has created a great communication and understanding gap between the couple. Actually my sister was a private bank employee while she got married and her husband worked in marketing department in some company but left his job last year January and used to sit in home and listen to his mother's words and used to harras my sister. When she was 6th month pregnant he behaved cruelly for silly reasons (like curry not good or if my sister answer's anything) and pulled her and threwed away by God's grace nothing happened. If my parent's question him regarding his behaviour he shows that result on my sister hence they stopped involving in their matters as per my sisters words. Now he started a medical shop in the month of August after spending 8 month without any job and due to transfer reason my sister resigned her job with her husbands permission . Now she is taking care of her husband and her son perfectly . His parents resides with him for 15 days for every 45 days. When they stay her in law's torture her mentally with the help of their son that is my sisters husband. Recently they left to their village that's it he started being moody and scolding my sister for no reason. Suddenly Yday because there was little bit less in salt quantity in curry he cruelly attacked my sister and hit her very badly in front of her 2 year old child. He also tried to attack with a wood stick but my sister some how escaped. But she completely got scared of her husbands behaviour and crying continuously. Now how we should proceed but my sister don't want to loose her matrimonial relationship? Sir/mam how to bring change in his behaviour without effecting their relationship Kindly answer my question.