Domestic violence and dowry demand

My query is for my sister. She is been continuously getting physically & mentally harassed by her husband (A government employee) from day one of their marriage which took place in 2010. He has been beating and torturing her every day for dowry and other petty issues since marriage however she was simply trying to adjust and save her marriage till date. He continuously threatens her by saying that he will kill her one day. Moreover, he also mentally harasses & threatens my mother (A single mother & a government employee) on phone by abusing and demanding dowry. He says that we did not give him anything during the wedding, though we did more than our stature. He also mentally harasses her by saying that she should not complaint against him or else she will have to bear extreme consequences. He tortures her by saying that if she complaints then he himself will commit suicide and blame her and her family for this. Please suggest something in this regard as it is only me and my mother in the family and we are in complete fear and mental pressure