My sisterinlaw is harassing my mother...

Hi my mother leaves with my only brother and his wife. My brothers wife is harassing my mother both physically and emotionally. She abuses my mother and says extreme bad things which are intolerable and only a sick person can do it. She snatches food phone or anything from my mothers hand.at times she pushed her and tried to hurt her. Day before Yesterday she punched my mother in stomach. I Sam married and stays far from my mother house. I have one elder sister also. My brother is not doing anythinh about it because he is afraid she will file a false dowry and harassment complain against him and my mother. Our relatives are not helping us as she threatens everyone about filing complaints against them.she also threatens my mother that she will file complain against me and my sister and our in-laws to ruin our married life.because of which my mother is not allowing us to interfere in the matter or take any action against her. We had agriculture land in our village.me and my sister and mother too we all gave land of our share to our brother around r4 years back. Now our mother don't have anything financially with hr. My brothers wife was always like this.. When I was unmarried and leaving in the same house.I also went into depression because of the stress created by her in the house. Me and my sister don't even go to their house from last 2 years because of her nonsense.my mother is around 65 years old.she walk with the stick and is started having problem in speaking also. Please advise how we can save our mother from this torture and can she be punished for her wrong doing.she is very smart in doing acting and drama and behaving innocent in front of people so that no one trust my mother .is there a way or anyone who can help us in capturing her when she misbehaves with my mother and threatens her of false complaints.