Rights of son on pagdi property

My father in law owns 2 adjoining rooms on pagdi which was purchased by his father in 1965. Initially owner issued two separate receipts but since 20 years he is issuing 1 receipt with no.as 12 instead of 5 and 6 earlier. We have old reciepts showing 2 nos. Now my brother in law stays there and intends to own the property. Though he has other houses he stays there. Can my father in law make a will and pass on the property to him as he is influenced by him. Can my husband get share in the 2 rooms alonwith my brother in law. What action my husband should take to safeguard his rights. Please advise.