Is development agreement & irrevocable GPA valid after death of owner

Hi, there is a land owner who got land of 2.5 acres and he had patta pass book for that too. later 2.50acres separated to 1.25 acres owned by his 3rd son and had patta pass book remaing 1.25 acres owned by him and they have been enjoying this property till 2005 . after father died . his wife and 3 sons( including 3rd son who had 1.25 acre patta pass book) enjoying the property as per Hindu law. here they ( wife and 3 sons) wrote a development agreement and irrevocable power of attorney with the a developer on 48 -52 % basis in 2008. but in 2013 wife died. Now my queston is ...is that development agreement and irrevocable POA valid even after death of one of the party out of 4 membrs of land owners -mother and 3 sons? coz The 3 sons said the agremnt no more valid and they wrote fresh POA which contains details of 29,74,000.00 value from that POA holder paid 29,64,000.00 as advance. remaining 6,000.00 only pending. I purched this propety via vijayalakshmi housing,gollapudi,VZD