How can mother claim property if uncle took control?

My maternal grandfather had four children: my mother, two younger sisters, and one younger brother. About 40 years ago, my grandfather bought a piece of land from his friend, "Party A," but never registered it. Recently, after my grandparents passed away, a quarrel arose between my mother and her siblings over property shares. My uncle (my mother’s younger brother) claims that he transferred all the property to his name through a gift deed from my grandmother before she died. We are unsure how he did this or whether it is a fake document. We also learned that my mother’s documents are still with Party A’s heir. My mother and her sisters want their share in the property, but my uncle is unwilling to share. How can we proceed legally? Is it possible for my uncle to transfer the property to his name without my mother’s documents? Can we claim an equal share among all siblings (3 sisters and 1 brother)? Please advise.