Division of property between 2 married daughters

Hello, Firstly i am writing this on my mothers behalf, she in her 60's now. so the matter is that I am looking for a legal opinion on my mothers rights over her parental Self acquired property . My late grandfather and grandmother has two daughters and both are married, but after their death my aunt's husband acquired the whole flat by breaking all relations with us by means of not keeping contact, not allowing us in etc. So i just wanted to know your opinions on this that whether filing a legal suit can give my mother her rights, my aunt is not even giving all my mothers certificates and documents (school certificates, ration card copy etc) which can claim that she is the younger second daughter. also my aunt's husband came in as a ghar jamai and started living there since a long time..i suspect this is what he was planning in his tenure there.. now that chawl is redeveloped and they have been provided with rooms in a society. Kindly give your valuable advise as in what immediate next steps should be taken to claim my mothers share...also i am not even sure whether my aunt has got any will acquired from my grandfather as he was alive during my aunts tenure there, and if he has a will how to get it to my notice. Thanks, Gaurav