Mariage_Divorce

After marriage with in few days my sister came to know that he is already married and divorced his first wife, which was hidden from us. When my sister asked him questions regarding his first marriage, he started abusing her mentally and verbally and used to fight with her every time, which we too listened.So due to his sarcastic and abusive behavior she had decided to start her life independently also planning to get divorce legally from him. After 2months days he and their family gave complaint that my sister is not signing divorce papers, in our hometown police station. And my sister is very afraid to go infront of him as he threatens her always.And that police SI wants my sister to come to station and sign on papers on in front of him.Even she too want divorce from him, she don't want to live with such a false person. But we want to clear it from court rather than signing before anybody. Because she is a girl, and if he does any harm for her or our family we all will be on road. And if she sign previously, we are worried that he may do some harm to her or my family. Kinly please help me, how to solve this...Please help me