What should I do in this situation? Furthermore, what do you do in such situations?
That's great that your mom is so supportive and willing to veganize her recipes for you. I've got both supportive and unsupportive people in my family, so I know how much of a difference it makes.
If I were in your situation, I'd definitely ask your mom to veganize the dishes that can be made vegan without changing the flavor significantly (my guess is, at least half of the menu could be adapted easily) and tell her not to worry about the rest. I can understand about not wanting to impose your lifestyle choices on others, because I feel the same way... but they would probably be happier eating something made with margarine vs. butter or soymilk vs. cowmilk, rather than have you at the table and not sharing food with them, so don't hesitate to suggest a few changes. This is also a good opportunity to introduce a few new vegan recipes to the thanksgiving menu that will become future family favorites.
Thanksgiving will be a creative endeavor in my house this year. I'm vegan (transitioning, almost there) and avoid bread, son is vegetarian, daughter is omni and not always very open-minded about trying new "vegan" things. (Last year, we were all omni) My mom just moved in with us last week, she's omni (but has been eating vegan/vegetarian quite a bit here because my food is yummy!) and allergic to wheat. It's just the four of us, so we don't have any other relatives to keep happy... but we haven't exactly figured out what we're eating yet, either. The focal point of our Thanksgiving feast has always been a large turkey and a lot of rolls with butter (all the things I'm no longer eating!) so this year will be very different, whatever the menu is.