Mental torture for my sister by his husband

My sister got married 3 years ago, beginning both of the couple are good, after sometime her husband started to mentally torture her by doubting her, not providing good foods, he won't buy even milk for her. When she got tooth problem he sent her to our house for the treatment. After 1 year we asked him to come to hospital for check up any problems for not becoming a parent for a child, he never used to come and ignore that. He makes my sister to go for suicide few times. Due to his torture she comes to our home frequently. Now he is saying that he doesn't want her in his life. For the past one year we are struggling with him ,we many time spoken with him but he is on his stand and saying more lies, he is saying if we take my sister to his home, he says that he will physically torture her and will kill her, we should not proceed legally. He is doing siddha treatment without any qualification. Among his 3 sisters 2 are impotent. Don't know what to do for her future and want to take decision