There's some time still before Thanksgiving to discuss menu with Sister. (I'm also assuming a fairly decent sibling relationship here, since there appears to be a lot of regular contact with Sister). Bluey has offered to provide staples. In hammering out the menu, I see nothing wrong with Bluey perhaps stating "I can provide the following abc staple items, but my budget will only cover these items for X number of people." If Sister wants a larger shin dig at that point, she has to step up (and pony up the money) for proper hosting.
Yes, my SIL and I have a great relationship (she refers to me as her sister, not an in-law), and she did listen to my offer to cook for a smaller group without being insulted.
I have read the above comments and I really appreciate the feedback. I do know that no one has the right to determine how someone else hosts or behaves in their own home. It might end up that DH and I have to make a decision not to attend if SIL just cannot scale it back.
Our offer sunk in! SIL has come to realize that she doesn't have the space, money, space, money, and oh yea, neither the space nor the money to host 30 people for Thanksgiving! She has decided that since none of the other people (outside her FOO) have ever invited her and her kids to dinner, holiday or not, she is DONE. She is simply not going to extend the invitation to anyone not "on the list."
DH and I had dinner at her house last Saturday night, and while we were waiting for her BF to arrive, we wrote up "the list," divvied up the groceries and came up with the game plan: I will arrive about 10am on Thanksgiving and I'll cook the basics, and she'll fill in the rest. I guessed correctly back in May when I first posted...there will be 11 people, immediate family + the 1 orphan who has become family over the years. She is sounding really excited about a lower-stress TG experience, and DH, his mom and I are thrilled because we just didn't like serving all these other people who treated us like they were at a restaurant.