My sister unsuccessful marriage

My younger sister is married to a person 6 months back. After some days of marriage he started complaining of my sister that she is not doing this work not cooking don't know studies. My sister is illiterate and is very simple. She understand any thing after a long time. She always remain under my mother. But we tell his husband all this things before the marriage when he came to see her before marriage. Now from the day of the marriage he is complaining and also talking with my family members disrespectfully and he is also mentally sick and very short temper. After 6 months we thought that this marriage of my sister will not last long. And nobody will remain happy and we also fear that in future he can harm and he can do any thing to my sister. We fearing that. Now my sister is in my house . Now we don't want to send her back to his husband house any more. And he is pressurizing my family to allow her to go otherwise he will do FIR on my family. I am his elder brother working abroad. So what should I do to not let her go to his husband house. Pls help me