My father has a land shared along with my other 4 uncles. This land they got from our grandfather.

My father has a land shared along with my other 4 uncles. This land they got from our grandfather. Grandfather is deceased in 1991 and he had got same land from his maternal uncle who had adopted him since his childhood. Grandfather's uncle didn't have son so very near successor was my grandfather only. So, apparently, grandfather's uncle had different surname and we have different surname. I will explain family tree below: Suppose A has two kids B and C, and B and C has there own kids B1 and C1, respectively. Also, B1 has kid B2, and C1 has kids C2, C3. And say, X is my grandfather who is nephew of B2, and B2 doesn't have kid. Now problem is C3 has claimed for X's land came from B2, since X is having different surname than B2 and B2 and C3 surnames of those are matching to each other. Also, he has (C3) filed case against X for fraud in land transactions and claiming for same. C2, that is C3's father, and X were good friends and neighbors and C2 never filed case against X for wrong doing. But greedy C3 is doing this. How should I proceed for this and what outcomes I may expect?