Marriage issue

Hello sir I would like to get a advice from you. My sister got married in Dec 2013 and now have a baby. Her husband is not supportive in any sense. When she was hospitalized for delivery he would not be coming to see her and if he comes he would fight with her. His parents are also torchering my sister, not physically but mentally. After their marriage we got to know that my sister's husband and his mother are suffering from mental illness. I would like to know whether he would be charged guilty if we apply for divorce and with whom will be the custody of the baby? I would also ask you one more thing. Before 2 months he had came home and taken my sister with him to another house where he had taken it on rent. There only 2 of them stayed along with the baby. But some days back again they haf fight and my sister came back home. Yesterday she needed some medicines and files of doctor for the check up of the baby so we had gone to their home. But when we reached there we came to know that he had changed the lock and put a new lock. So we decided to go to police station and complaint against him but police said we cany take him in custody and you can go and make a duplicate of the lock and take whatever file you want. Police also said that as he is a mental patient we cannot do anything. So now we are confused what to do. My sister wants to take divorce but we think that as he is a mental patient he would easily escape from the case and no money would be given to my sister. So please guide me what to do now? Thank you. Armaan