Harassment to sister by Mother in law, husband, husbands sister

My sister married to a guy before 1 year & 3 month. After 2 months monther in law is started mentally harssing her on petty petty issue in cooking such as less salt in meal, ovecooked meal on such things she had gone to extreme n started harassing her for entire day even her husband is under her influence n he also sarted harassing her since last 11 months of 15 months marriage he is not talking with her & just fighting daily. 2 times he slapped her also. Mother in law is harassing her in front of other people also n due to all this my sister started loosing her confidence. She is housewife n not highly educated. Now she is at saturation point n fed up with daily disputes since no one is in her side n all people from husband to his sister's n mother in law harassing her. I told him if u don't stop all this we will go to police station. But they are not stopping it. Her husband, MIL n his sisters are forcing her to leave house & harassing her so that she will leave house on his own. Please advice me what steps I have to take.