ancestral property sold without sale deed

My uncle had sold ancestral property ( agricultural land) in 1985 signed on sale agreement on plane paper, no sale deed was done as there was disput between them. buyer did not agreay to cancell agrement and continued to use land for harvesting forceably. Now my uncle wants to sell the disputed property which is ancestral property in which my father has equal share. Request for your sugessation on legal consequence and getting my right as grandson