Procedure of transfer of ownership of flat

Hello Sir, My father is holding flat (property) in mumbai....One is in his name and other is in name of his sister. My grandfather had taken flat in name of daughter as my father was minor..Now father's sister want to give flat to my father back and want to transfer in my father's name. Another thing there is no society form in building and still it is in goodwill ( pagadi).. My father pays maintainence of both flats regularly by cheque from his account..Could you advice me how should i carry on for procedure