My brother brought his girlfriend to our siblings-only weekend and says the rule is childish

Once a year, my two brothers, my sister and I rent a cabin for a weekend. No spouses, partners or children. It started after our parents divorced, when we realized we were only seeing one another inside larger gatherings where somebody was hosting, supervising kids or leaving early.

The rule is not a judgment on anyone’s partner. My wife likes that I go. My sister’s husband jokes that it is our annual meeting of the board. Mostly we cook, walk around, play cards and talk in the shorthand of people who remember the same broken dishwasher.

This year my younger brother arrived with his girlfriend of eight months. He had not mentioned bringing her. The cabin had four bedrooms, so there was space, but that was not the point.

He said they were already in the area for a wedding and it would have been rude to leave her at a hotel alone. She looked uncomfortable while he explained this, which made it impossible to challenge him without making her feel rejected to her face. We said hello, changed dinner plans and tried to be normal.

She was perfectly pleasant. She also changed the weekend. We skipped a conversation about our dad because my sister did not want to discuss his drinking in front of someone new. My older brother stayed in host mode, asking questions and filling silences. The girlfriend went to bed early on Saturday, and my younger brother followed her. Nobody did anything wrong, but the thing we had protected did not happen.

Afterward I told him I was upset that he had changed the guest list without asking. He said the siblings-only rule is childish now that we are adults with serious partners. He also said excluding her sends a message that we do not see her as family material. I asked why he had not raised that before the trip. He said he knew we would say no.

That answer bothers me more than the surprise itself. He made us absorb a decision because he knew courtesy would prevent us from objecting once she was standing there.

Our sister wants to keep the weekend and tell him clearly that anyone who brings an uninvited guest next year will need separate lodging. Our older brother thinks the tradition has probably reached its natural end. I do not want to freeze our relationships in childhood, but I also think adult siblings are allowed one small space that is not automatically expanded to every partner.

Is the rule still reasonable? And if it is, how do we defend it without making his girlfriend feel like the family voted against her personally?