My Father Land Is Going To Be Occupied By My Aunt

My father has a land and he sold little part of it to my mother's elder sister who is unmarried. She has build a rent house in it and collects rent every month since many years but the total land is in my father's name and there is no proof that the land belongs to my aunt. But now my aunt's nephew who is a lawyer wants the part of the property as he looks after my unmarried aunt and he has warned us that he'll give a case and threatened us that land cases lasts long and it can cause a lot of trouble for us if we dont give him the land.