There's a way I can understand Larry's (possible) point of view: His third marriage ended, and he's with a new girlfriend. After three marriages, I wonder if Larry is the type of person who jumps into relationships with both feet and little thought. In his mind, NewGF could be significant even if they have only been together for a week. They could be discussing moving in together or getting married (although she is not presented as a fiancee at this time). Whether relationships are temporary and disposable to him or whether he falsely believes each time he has found a forever-wife isn't clear.
By (presumably) the Aunt's wording in her scaled-back invitation, in the past Thanksgiving has included girlfriends, boyfriends, family friends, and lonely neighbors. Larry may be envisioning a hierarchy of sorts. Why shouldn't his significant other be "higher than" a group of maybe: teenagers' boyfriends, neighbors who had no other plans, a family friend who is in town, etc? It also may not sound fair to him that he must come alone while his siblings get to bring a spouse and three kids. It may not sound fair to him that he can't have Thanksgiving the way he wants all because Aunt is getting old.
I can appreciate -- or at least have compassion for -- that point of view.
Audrey Quest helped me see that Thanksgiving could be viewed as either "our event at Aunt's house" or as "Aunt's event". Aunt seemed to have been very welcoming in the past, so everyone must have felt bit of ownership in the gathering. Aunt, as host, changed the rules this year, and that can be off-putting. Any of the guests could be thinking "That's not fair! That's not what WE have done for the last three decades!" I don't believe they would be wrong to have that reaction. *But* the Aunt has every right to limit access to her home to a guest list of her choosing, even if it is different from her choices in other years. The lines she drew were arguably within the norms of etiquette. The fact that is was a change from the past is hard to swallow, and it may be the trigger to lose some family guests, but it was not rude.
The invitees have the choice to say "thank you, no" and make other plans. For whatever reasons, they didn't. Maybe they adore her so much that she can do whatever she wants. Maybe they're happy to have a family-only gathering for once. Maybe they feel bad for her as she's facing such a major life change. Maybe they think "we'll just put up with crazy old Auntie for one more year, then we're free!" It is even possible that she is the family bully and Larry's dad only spoke up on her behalf to make Larry keep the peace by not upsetting her.
In any of those cases, I have the same conclusions over and over:
Aunt is not rude to limit the guest list in her home.
Larry is OK to be upset by the change.
Larry only gets to accept or reject the invitation.
Larry was rude to bring his girlfriend.