Property dispute between siblings

We have flat in kandivili west 1/bhk which was purchase by my father in 1973 saleded was done on his name we 5 siblings 4 sisters and 1 brother i am 2nd (brother) after my marraige in 1990 my father expired without doing any will so after my mother name came in the flat as nominee then all 4 sister got married 3 are out of country 1 is in andheri and due to some internal family problem i was staying in ahmedabad coz my n my wife are doing jobs and since my mother expired last year before her demise my sister cut of all relations with me with no reason and pressurised my mother to do nominee share of flat and for me they have written only 5% others all 4 have kept in their name now the flat is lock all the documents my father's orignal saledeed ,bank ac which is joint name of me and my late mother documents locker key all this my younger sister have with her and not willing to give any thing i have written official letter to society regarding the matter soon after my mothers demise infact my 4 sister cruelly dint allow me to do the last rituals of my late mother so how can illegally get my rights and legal heir certificate please help me