How to sell ancestral land in name of father in law and his sister?

In reply to your query, we would like to know where is the land situated and is the land agricultural or is the person merely using it for agricultural purpose, as depending upon these the appropriate proceedings can be initiated.

Since the share / portion of your father-in-law and his sister is also not specifically demarcated, a suit for partition can be filed, whereupon your father-in-law's share can be specified and thereafter he can sell the same to a third party, if he so desires.

Further, the relations between your father-in-law and his sister are also not clear from the question. The person in occupation can further say that he is in occupation of portion of his mother.

Hence, we would require more details to ascertain the way to secure your father-in-law' rights and interest in the property.