How to divide mother's property among legal heirs?

my mother is +90 yrs.My father had acquinted residential property,died in 1984 without preparing a will.My mother was nominee and flat was transffered on her name ,since 1982. My elder sister is unmarried & taken all responsibility of my mother while elder brother is also residing with my mother with his family members.Elder brother is not takinen any responsibilities of mother .He is paying routine monthly expense Rs. 10,000/- to my mother as his family also residing with my mother.He wants to grab entire premises on his name so he everyday giving mentle tension to my mother & quarreling with her & compel her to give his share from this property.he is giving threat to mom that you are nominee so you cannot write will and you cannot distribute this property. please give us advice so that my mother remains in peaceful way