How can my sister get a divorce from an abusive husband?

Please guide us on how to get a divorce. My sister has been married for 8 years. Her husband physically abuses her and is mentally unstable, making irrational statements and actions. He has tried to kill her multiple times. My sister has been living with us (her parents) for 2 years and 6 months. We have called him and asked about the situation, but he verbally abuses us. We are six sisters, with no brothers or father, only our mother. We are very distressed and need help. He refuses to give her a divorce. The case is currently in court, and my sister is unable to speak or make decisions. What should we do?