How to deal with unfair occupation of land by family member

My grand father had 4 sons, one of the son (younger son) has occupied complete land. Our grand father had made an affidavit stating that after his death the land should be distributed equal among four sons. However after his death, my uncle(younger son) forced our grand mother to sing white paper and with that signature, he made land patta on his name. How can we get back our share in this case.