Gpa validity

Hi, We have an ancestor property of 1.7 acers, my grandfather has 3 son, my father is 2nd son. The property was made registered GPA to third person by my grandfather, my father and his young brother, but the elder brother did not entered in to this transaction. This happened in 1994. The GPA holder made layout and sold sites based on the above GPA. There are already some sites which are constructed and some sites are still not constructed I came to know from our family the GPA holder has cheated and used the situation of my aunt marriage to get this signed with small amount. Question. 1. As it is ancestor property, with out having all coparcener(elder brother) the GPA executed is valid? Or it is void ab initio. 2. My grandfather died in 1997, will the GPA get terminated with one executor death. 3. Is there any legal chance where we can put a case on him. 4. Elder brother of my father has rights but he is not educated and also same with his son. It is hard to make him understand the rights what he has. Can we fight on behalf of him. Please guide me , how to go forward.