Relinquishment Deed

Dear Sir, I am in the middle of buying a house in Ahmedabad. The owner is no more. He has not left any will. Now, the share certificate of the property is the name of the deseased and his wife (alive), allottment letter has name of wife, municipal tax in name of wife, tax bill in name of wife. But there is no sale deed as property is of 1993. Initially when they bought this house the name of the same was different and then the scheme got bifurcated and a new name has been allotted. There is document for the same (Society Registration, Development Agreement, Order from Registrar Office). As the sale deed is of soceity, we asked for Key Plan and Unit Plan approved by Competent Authority. I have submitted all this document. The loan agent has requested to get a Relinquishment Deed done as The Deseased has a wife, 2 sons and a daughter who has been married and staying at a different location. I am coordinating with 2 sons for the property. I have also met their mother (Deceased's wife) and even she is willing to sell the property. I also got an NOC from Society of No Dues. The problem now is the agent has asked Relinquishment Deed and the 2 sons are telling, they would add one paragraph at the time of Registering House in the agreement that the sons and the daughter would not claim the right of the property in future. I am totally unaware of this. I told the seller to make an Relinquishment Deed as it is mandatory at Subregistrar because there is no will stating the property owner. Please guide me.