Family Property Dispute

Family - Father F, Wife W, Elder son ES, Younger Son YS 1. F died intestate 2017,has self own flat Mumbai. Flat was purchased on 2001. 2. F helped ES financially in 2007 to purchase another flat in next society on ES name. 3.Fathers Society went redevelopment in2018 by taking noc, indemnity bond no 34,35 & letter of consent for redevelopment frm all three heirs.And making YS member. 4.now new building is ready & ES claims his share in flat. ES vs W, YS disputes started after new building constructed. 5. Builder & society says this flat will be registered on YS name.As YS want proper release deed & legal heirship of flat YS is ready to give ES his share monetary market value. W is ready to release her share without any monetary value. Question- A. Is legal heirship compulsory in this case? B. YS is worried if ES say he don't want money and not releases his share after getting legal heir certificate then it will be problem. YS & W not trust ES. ES can change his mind by not releasing rights after legal heir ship. So what should YS should do? C. After release deed can transfer of old property be made to YS name? D. What YS should submit to society or builder after transfer of old flat done & make old flat share on YS name? E. Society is having triparty agreement to nullify stamp duty of previous flat area and they include YS name on it as a member. Should YS sign it?