my sister is harassed by her husband

My sister is in deep trouble. She has lost her independent. She is not free to talk to me or father and mother. She is not free to meet us from past 2 years. She is working and earning good salary. She is paying home loan emi. She had 2 kids She is living in partly joint family, her house is 50 meter away from her father in law and family which is very large. Financially she is strong to take her decision by own. But due to some strong threat by her husband she has lost control of herself and forced to do whatever her husband wants to do. she is on deep stress. Keeps on crying whenever get a chance to meet. But due to some unknown threat by her husband she is not able to say anything. (May be is a death threat of her parents and brothers, because her husband has links with wrong guys) Now who is authorise to take help from our law and hoe. My sister will not do this. I want her dignity back. But she is not in state to help herself. Her life is worst then animals. Her husband has full control of her. Please help her to get out of this.