Regarding suggestion against will.?

In my village we have a hierarchy agricultural land which was gifted by the father of my grand father to him and his brother. My grand father have two sons my father and my uncle. My grandfather have four nephews. One of the nephew exchange their land to my grandfather. But on paper we show that we purchase the land likewise the nephew of grand father. But after the death of our grandfather we got to know that he made a will in favour of the wife of my uncle. In which he gave the ownership of the land to my aunt. Plz. suggest me what I do in this case. How can i challenge the will and how i prove that my grand father is not the authorized person to make the will coz, it is hierarchy agricultural land. After the death of my grand father all the rituals done by my father (elder son) and all the bills paid by us.