1. It is really pity that your brother seeing all these is keeping silence or is not coming forward to rescue her own mother.
2. Ask your mother inform the local police about such threats and intimidation.
3. Such cheap people must be dealt with sternly , preferably, with the intervention of law.
4. Alternatively your mother can lodge complaint with Senior Citizens Tribunal.
5. She must raise protest. Laws are in her favour,So there is nothing to worry for her.