As a grand son of my grandfather can i get possession of my flat

Under the right to information 2005..i have got info about my family room situated at mumbai,byculla. but the developer who redeveloped my flat not giving possession..becouse some of third party approach him with the sell agrrement & generl power of attorny about the same room which is not stamped or register. my grand father was owner of that room.he is no father also died. now here builder reject to give possession to me and saying that your grand father old his rights by illegal way he could not able to sold this room up to 10 yrs and ..your gradnfather by power of attorny and sell agreement tranfer said room to that third party which is illegal and its violate my clause and rule 33(7) of developmnt authrity of mhada. after that mhada officer also give order to that developer that this sell agrement and power of attorny is invalid due to nor it stamped or register. and give possession of room to the owner only. still developer not giving possession.and file wrti petition against the order of the mhada officer. plzz sugest me can i get this property as legal heirs of my gradn father? and this sell agrement and power of attorny is nor stamped ntot registerd.