Partner and I got together a few months before Thanksgiving. Our first Thanksgiving we spent with our respective families, I didn't even think of asking to bring her along, the relationship was still pretty new. Our second Thanksgiving together her maternal grandma was coming into town and wanted to take her family out for a fancy Thanksgiving dinner. The guest list was Partner's mom and her boyfriend of over a decade, Partner, and Partner's brother (her sister was invited but lived out of state). I was not on the guest list. I didn't even think twice about not being invited. We'd been together a year, but we didn't live together, we weren't engaged, we were just dating each other.
So two years ago we've moved in together, we've been together three years, and grandma comes back to town and guess who gets an invite? Me! And it was a very fancy dinner. Why? Because we were now an official social unit. I was (and am) part of the family now. But way back when? I was the girl her granddaughter was seeing.
So yes, Larry very rude Aunt has every right to greet them at the door with "Oh, Larry didn't you understand, this is family only? I'm so sorry dear, I thought I made this clear to him, I'm just not able to host any extra people this year. It's nothing personal", because it's not. Family only, means family only. Now in my family family only means anyone whose willing to board the crazy train express. But in other families it means, blood, marriage, or another long term commitment made. Not, this is Susie, we've been together 3 whole weeks!