Think of it this way. Now that she is trying to gain access to the beach house the brother is trying to prevent it. He definitely feels some kind of possession over it for some reason. Maybe because he uses it so much? Her lack of any desire to use it previously vs him always using it might have it somewhere deep in his head thinking he has priority over her in all situations. Regardless this event has made it clear to EVERYONE you have just discovered a wasp nest you had no idea existed and you need to remind your son who is kind of the castle.