Release deed vs death will

Hi sir, My grand mother had agricultural land in her name which is granted to her by government(darkhas land), she had 2 sons. My father did not take share in ancestors property and he was doing agriculture in darkhas land which was in my grandmothers name. Family members had agreed to give this land to my father. So My grandmother made will in my name for the same. Just before 3 months to her death my fathers elder brothers son took her to sub registrar office and executed release deed for the same property in his name so the latest EC has his name. My grandmother is illiterate and her health condition was also not good during that time so no one got to know abt this. After 6 months of my grandmothers death we got to know about this. Now my cousin is refusing to transfer the property to my name. We are in the possession of this property for more than 30 years but RTC is till in my grandmothers name. Now what is the best way to handle this? In this case release deed or will deed has more power? IS there any way I can fight legally to get this land?