Property sold without informing the owner

It is a story of nearly 40 years ago. My father was married three times. I am one of daughter of him from his third wife. I have my own one sister and two brother. My mother i.e. my father's third wife still alive. My father got married to my mother after death if his first two wife. From first wife he had one son and one daughter (died), and from second wife he had two daughters. Now come to my problem, I came to know few days earlier that when I and my own sister and brothers were minors in age that time my elder step brother (from my father's first wife) sold a land to a person without informing any other else. During this deal he (my step brother) actually had or hadn't take my mothers concern/permission, I don't know and my mother also forgot. Now to whom the land had sold he is trying to sell it to other person and from that reason we came to know about it. So my question is that what is the action I should take now ? Is there any way to return back the legal custody to my mother or to us ? Or any thing else I should follow ?