Hmmm...wonder how he'd feel if you and the kids got in a car accident on Thanksgiving (heaven forbid, of course). Probably a lot worse than if his mom had a heart attack. I know he's between a rock and a hard place, but decisions are the stuff of life. Either one is a horrible thing to happen, but he can't always default to mom and make his family bear the brunt of that decision.
My family is in Canada and Kelly's family is here. We often can't go to see my family because they are too far (3 flights each way or 33 hours driving each way). Kelly's family is a 2 hour drive. Therefore, we make decisions. Thanksgiving with one family, Christmas with the other, and if one of those can't be with the "other" family (generally mine), then we spend it by ourselves. I don't think it's fair that we have to go to his family by default just because they're closer. It took some time for his mom to get used to that but I think she has learned to live with it.
If my mother offered to fly me home alone for Thanksgiving, unless there was REALLY good reason (i.e. Kelly was in Cancun on a business conference or something...) then I would say no. Don't get me wrong, I love my parents dearly and I know they would never do that, but if they did, I simply could not accept.
In a way I feel you are a saint for putting up with that nonsense, but in another way, I feel you're not fighting for what you and (even more) your kids deserve. You're trying to be nice and loving and caring and letting him make the choice, but you're getting walked all over in the process, and you're giving the message that you and the kids are less important than his mom. Every other year - that might (possibly) be fair. Every year...no way....