How to stop sister from mentally harassing my mother

My elder sister mentally harasses my mother from past one year. She uses taunts, abusive language to humiliate my mother. My father passed away five years ago and we have a court case going on since last four years. My sister in not supporting family from last two years even though she is well educated with degree in engineering. I guess she has depression which she sometimes uses as weapon against my mother. Me and my younger brothers are also not on good terms with her because of her behaviour with our mother and also she use to fight with us too. So I want to know if there is any law to control my sister for her behaviour at least i could stop her mentally harassing my mother.