Mother being subject to cruelty by brother and his wife.

My middle brother who is married lives with parents. Other two brothers including me live overseas. And sister who is eldest is married and live in Lucknow too. The issue has started as my brother refuse to work and so do sister in law. They provoke relatives and known people to harass other family members by spreading rumour. This brother threaten all siblings and use rough body language against mother. He has been pressurizing parents to give all property and pension control to him. He and his wife spends money on themselves and when mother object they tained her as mental, and when sister intervened they spread rumour against her that she is having affair, fought with her on many occasions, when I as a brother tried to take a stand then he picked up physical fight for no reason with mum as witness. Father has been greatly influnced as 80 percent property under his name. He is rude to even mother and other siblings. What to do ? I need to go back and work but need to save my mother.