Nomination law

Seniors, My mother nominated both my sisters in her flat agreement at Mumbai. (She brought this flat from share received from my father own property after his demise) she then told verbally to both my sisters that now she wants to give equal share to both my son as well (we are 2 brothers and 2 sisters. All of sudden In short illness she passed away last year. Then both my sisters transferred that flat without our knowledge and gave in rent. I have send them notice for our share but not received any revert from them. What should be my next step? I sent notice to both my sisters, building secretary, local police station and Tanent.