For those who are saying, effectively, that Aunt had no right to set her own invitation list because Thanksgiving is an important holiday, so she HAD to issue an invitation for family and whoever they would like to bring, would your opinion change if the event was a wedding?
After all, Thanksgiving happens every year, weddings happen once in a person's life (or only a few times).
So, let's recast this as a wedding. Aunt is getting married, and, for whatever reason, wants a small ceremony. She issues invitations to family members only, but including spouses using the 'social unit' rule. Larry wants to bring his new girlfriend. Two people tell him not to, as the ceremony is planned to be a small one. Larry ignores their advice, and brings GF anyway. Aunt goes off on him.
In this case, I think everyone would admit that Larry was rude. I don't see the actual situation as being any different.
1. There is no evidence that the GF was a serious one BEFORE the Thanksgiving event, as such, the social unit rule does not come into play
2. There is no evidence that anyone is censuring Larry's morals. His behavior in bringing an uninvited guest when he'd been warned not to? Yes, absolutely.
3. There is no evidence that not attending would have had any impact on Larry. I can't see how it could be worse than what he actually did.
Aunt offered to host a gathering she could manage. The notion that that it was Thanksgiving meant that she HAD to invite all and sundry whether she liked it or not is a red herring, I think. Frankly, Thanksgiving is a holiday, but not one of such overwhelming importance that a person CANNOT be separated from their SO for a few hours. An invitation is not a summons. Someone else could have hosted. Larry could have gone elsewhere.
I am firmly on Team Aunt.