If your father-in-law is all alone, why do you refer to his house as "their" house? Does he have dissociative identity disorder? If so, I would say stay where you are because "they" might cause problems for you and your wife.
Because they all use to live there years ago when his kids didn't yet marry someone. This brother in law lives there as well when he was younger and my wife also lives there when she was in high school. Its their house which was inherited from their grandfather and from their great great grandfather. Just their house even if they don't live there anymore.
I even have reason to believe my wife and this brother in law will fight over this house.