who is the legal heir to unmarried father's elder brother?

Legal heir to unmarried father's elder brother: my grandfather and grandmother died ,they has 2 sons (my father and father's elder brother).my father died 20 yrs back. my father has 4 children. my mother is alive. my father's brother ,he was unmarried and he died recently.he was the gaurdian of our family and property till death .and he has one house which is on sale aggrement .And the sale aggrement time period expired .so the opposite party file a case against our father's elder brother before 1 month of his death.my mother requesting to opposite party by returning their money and getting back our house,but opposite party refused it and trying to register our house.they trying to say that we are not legal heir .In this circumstances how I will handle the situation. Please advice. 1.who is the legal heir for my father's elder brother? 2.how can i get back my house ? 3.to whom the house belongs to? (whether to opposite party or to us ?)