Legal ownership/Share of property

My grandfather passed away recently while my grandmother is alive. The house my grandmother currently lives in was built by my deceased grandfather. He passed away without a will. They have 2 children, my mother and my uncle. My uncle was given legally in adoption to my great grandfather' brother when he was young as they didn't have any children. They passed away a long time ago and their property was given solely to my uncle. Now my uncle's wife is claiming a share to my grandfather's house and she is pushing my grandmother to get a heir certificate for my uncle along with one for my grandmother and sell the house to give her a share. Are we legally obligated to give a share to my uncle? We have a copy of the unregistered adoption paper and are trying to get the registered copy as well. But if we can't get the copy, will it affect a share of the property? Can my uncle get a heir certificate for himself and sell the property without our knowledge? How would we stop him from doing it?