"My MIL comes from a culture where time is flexible and no one gets bent out of shape over an hour to two. It's assumed your going to be late. I work with people from this culture a lot. It took me years to get passed my annoyance about meetings never starting on time. An 8am meeting means they show up in the conference room at 8:15, visit, grab coffee, and start the meeting around 9 if I'm lucky."
My ex was from a culture like this, and I also work mainly within that culture. We don't allow it with our own staff, but for external meetings with clients etc, yep I build in an automatic hanging around time until they turn up. It really is inherent in the culture and they even call it 'Blue-time' as a joke. On the personal side though, with my ex, it was an accepted thing but when there were meals involved generally there wasn't an expectation of the meal being served at any set time. The meal was prepared and kept ready, then people were served and the food reheated for them as and when they arrived. We rarely ate as a group with other people - it was all done in dribs and drabs throughout the get together. I realise that's trickier with something like Thanksgiving when you all want to sit down together, but for more informal meals you could try that approach.