My situation was not exactly the same, but there are some striking parallels, so I thought I would share.
My now-husband and I were very good friends throughout college. I visited his house once with a group of friends, but didn't really know his family at all. Two months before graduation, he broke up with his girlfriend of 4 years, and he and I started dating. We knew right away that we would get married, but of course nothing was official yet. Three weeks after we got together, his grandfather (who I had met once) passed away. We agreed that I would not attend services for his grandfather. It would have seemed too much like I was intruding on family time and trying to insert myself too soon. A grieving family shouldn't need to feel pressure to properly welcome and include a newbie. (obviously, I would not have made it all about me, but his family didn't know me yet to know that I wouldn't)
Clearly, Thanksgiving is different than a funeral. It is a national holiday that people expect to be part of some celebration. And Elderly Aunt isn't dead, or even at death's door. But, because she is looking at a very significant shift in her life, I think the whole family was aware that this Thanksgiving might have a very different feel; while not being mournful, it was not as likely to be as cheery as normal. I am used to holidays being memory-making events, so any new people coming in are adding to the festivities and merriment. In this case, however, I imagine there was more reminiscing and memory-sealing taking place. Adding a new person to the mix, who has had zero interactions with the family, would have completely changed the dynamic. Conversation might have been pleasant, but certainly not intimate.
Larry was rude for trying to change the nature of the event, and doubly rude for ignoring his hostess' request.