Issue regarding Ancestors land given for development to builder

Sir, i need some clarification regarding few issue. 1. My mothers ancestors land in NAVI MUMBAI have give for re development to a local developer /builder to build a building , thus my mother and her siblings made an agreement with the builder to build a building on it and the nos of flats will be divided equally (50:50) between the builder and the owner (my mother n her siblings) , but now the builder is demanding money on the name of GST for registration of the flat of our share , does it right or he need to give us final possession by giving key of the flat n bear all expenses ??? 2. My mother requested the builder to make a joint registration of her share of a flat i.e joint registration of my mother's flat by her n my (daughter s )name , earlier he was agreed but now he is denying that joint registration of my mother's flat is not possible as this is re development property , sir i want to know about the rule n fact that is it possible or not ??? Thanks n regards Ashwini