get back my farm which is in possession of grand mother

There is our ancestral property of 3.5 acre, in possession of my step grand mother , now she has given that on annual rent to someone since last 2-3 year, now a days we want that property back to us and whatever rent she is getting. We are ready to give herore than that, but she is refusing to us. She want to continue with that person. But we fear in future he will be acquire that property and he will convert it in his name. Can anybody guide me what should I do?