Get back my father’s share in my grandfather’s property

Dear sir, My grandfather had 2 acres of land. later my grandfather died without a legal will or doing partitioning. my grandfather had 4 sons and 4 daughters. And after my grandfather’s death in 1991 approx. the property came to my grandmother’s in charge. All sons and daughters are married. My father is the eldest son of my grandfather and I am his only son. My father’s health was in critical stage in 2006 and my grandfather’s son and daughters and my grandmother decided to make share of the land among sons. The three daughters agreed that they do not want the share in their father’s property and signed the agreement. One of the daughters was not present so she could not sign it. The agreement has 1 original document and three duplicate copies. The original copy is with the 3rd son of my grandfather who is given the house of my grandfather. During this agreement I was not present with my father.The first copy of the agreement which is suppose to be with my father was kept by the third son of my grandfather. In the agreement the entire property i.e 2 acres is shared among 4 sons of my grandfather (Aprox 45 cents each). First share of 45’ cent belong to my father. But they added my grandmother’s name also in my father’s share and mentioned that both the party that is my father & grandmother has a joint possession on my father’s share. But rest of the 3 son of my grandfather has only their name in their share. Almost a year after my father expired (in 2007).After my father’s death the keeper of the first copy of the agreement (3rd son of my grandfather) said that it will be given to you after the grandmother’s death. After 4 years my grandmother also expired. Now when I am asking for it he is not willing to give it saying that I will not give it to you. You do not have any right in this property. Sir how will I get back my father’s share of my grandfather’s property since my father and my grandmother has already died. Against whom should I file the case?