How to go about transfer of property using official sales deed

Hello , My brother has purchased a sra flat in mumbai through unregistered POA as 10 year was not conpleted .since then we are residing there with all originals papers of the person who got alloted including share certificate..allotmrnt letter etc. I guess those unregistered sales aggrement is void by laws nor we can now pay the stamp duty . Now the seller and my brother both have passed away and I want to sell the flat. The legal heir of the owner is still alive and she is ready to coperate in sales. Ideally building has to first transfer legal heir of the owner on the share certificate only then the she can sell and obtain noc from building. But we are confused as once society transfer her name on share certificate she can also bypass us before making the actual registered deed for sales. So how can we safegard our interest for time between transfering her name on share certificate till making registered sales deed with my buyer. Let me know if there is any other way...