Final decree for property

My mother has acquired ancestral property through her mother. There were 22 initial shareholders (my grandmother being one of them). Since my grandmother passed away she had 3 daughters among whom the properties are to be divided. There is a property near hitec city in which my grandmother was allotted 9 acres in her share. The property has not been sub - divided between her 3 daughters. Now 2 of the sisters had done agreement for their alloted share (6 acres). But the party who bought their share dis not subdivide the share and now claim that all 3 L.R.s signed their shares with him. My mother never signed her share with anyone. When we confronted the buyer he is claiming it was signed. When we asked for evidence he is refusing to furnish copy of the agreement. Further it has come to our notice that he approached the high court of ap and got the final decree in his favor. We would like to know what course of action we need to take in this regard. How can we apply for final decree on our won for my mother share. The partition of entire property was approved by the high court of ap back in 2009.