How to add names of heirs on the property

My Great Grandmother who was a Catholic by Religion passed away several years ago without any heir as she was unmarried; however had adopted my Grandfather (No Adoption Papers) who has (5 Sons and 2 daughters). Even my Grandfather has passed away leaving behind the property which is still registered in my Great Grandmother’s name. It is very unfortunate that none of us are able to locate whereabouts of her death burial place and hence unable to find death certificate of my Great Grandmother. We have the possession of the entire Agricultural property which is in Mumbai and is currently used to cultivate crops. We have tried searching in all the local churches to see if there is any record of her burial however no success. Also, recently 2 of my uncles have passed away and their children are asking for their share in the property. What is the procedure to add the names of all the parties (Grandfathers children - 5 Sons & 2 daughters) on 7/12 who currently hold a possession of this property?