It was rude, there's no question. Bad form, absolutely. Uncomfortable for everyone, completely. Empathize with your reaction-easy.

Here's my take. My wife is FOREVER telling me, "You don't know their story," when I get upset at someone, friend or stranger. Whether it's road rage at a hesitant driver or the action from someone else. You couldn't have known what led the parent to decide bringing the sibling was acceptable. Any hundreds of reasons; they could be financially strapped, and they know you are not. The sibling kid could be bullied at school, always left off teams, never invited to parties...you don't know. Maybe it's the kid that was invited that has perhaps dealing with an issue and the add-on sibling was there to look out for them? You can't have known.

If you had a chance to repeat the situation, perhaps you could have turned the scenario on its ear and demonstrated compassion to your child and even taken it as an opportunity to teach your child empathy, inclusion and kindness. Not that your child had any idea something was off, but you could have made the extra child feel extra accepted. Everyone would have won, and I'd say you as parents would have felt an emotion other than frustration at the end of the day.

I don't know. Maybe everyone is only worried about themselves. Maybe that's the new normal.