Dear Client
Yes your brother in law can claim the share from your father in law property.
But If the registration of flat has not been done yet the you can change the name of owners as your husband and your mother in law and take a will from mother in law in which she writes her share in flat on your husband name.
And if not the your brother in law in claim 1/3 share from your father in law share which is 1/3 of total property.