Property title

My greatgrandmother had made a will leaving our ancestral house to her 2 grandsons sometime in the 1940s.  Later in the 1970s or 1980s, my great-grandmother's daughter (my grandmother) made a will in favor of her daughter (my mother). I am not sure not which of the wills is considered valid.  Kindly guide me as to whether the will in favor of my uncles is valid or the one in favor of my mother is valid. 

Thanks in advance.

Sri