Ownership of flat in mumbai

Three years back my mother died (aged 84) and she has made nomination in my sole name for a residential property in mumbai. She didn't choose my other brothers (we are 4 brother in total) in nomination because they left home long back and was not willing to look after her. She hasn't made any registered will but I have her hand written letter stating she desire to give all her assets to me. When I claimed my nomination to society 2 brothers gave an objection letter to society that it shouldn't transferred it to my name and one brother has no objection to it. Society replied back to ask them to get it through legal course and they have added me as a nominal member in societies book. Since I am also aged 62yrs and due to recent pademic situation the world is going through I wanted to make my will and ensure I don't repeat the mistake done by my mother. How can I ensure my two kids get the complete legal ownership of the flat and/or selling rights in future so that they dont face any issues after my departure. It's been 2yrs my brother's have not filed any legal suit neither are ready to provide NOC or release deed.