Wait, what the hell! Parents can only use the “your more mature” excuse for so long. Once everyone reaches a certain age everyone should be equally mature and responsible for their own actions. If BIL is old enough to rent a space, then it is obvious that everyone is well beyond that age. BIL isn’t mature because your in-laws don’t expect any better out of him. He gets to throw a tantrum and act entitled to your property because his parents let him.

Unfortunately, I have seen parents who have one difficult child, or one child who does something really shitty and they will ask the other good and responsible children to take it on the chin, because the parent wants all of their children together no matter what. Your in-laws find it too difficult to deal with BIL. He is their stubborn one, and they don’t want to fight with him.

It is completely reasonable for you and DH to ask them to stay out of the issue with BIL. Your husband should tell them that neither you or him will be discussing this issue with them anymore, especially during the rest of your pregnancy. He should tell them that BIL’s behavior has stress you out enough, and the two of you are no longer interested in entertaining BIL’s childish behavior. If he isn’t mature enough to act like an adult that isn’t your or DH’s problem, and it’s time BIL suffered some consequences for his behavior and grows up.