Iris, I think that I already posted several times before this, but your thread keeps drawing me back.
I just finished reading the Wikipedia entry on coeliac disease. This is not just some little quirk in people's heads. This is a real condition, which thankfully can be well controlled by diet.
Didn't you say that your SIL is a nurse? She should understand this. I think that your brother is rather clueless, that he looks to her to deal with family and probably friendships, too.
You mentioned other reasons why you don't often socialize with these people. I am very sorry to say this, Iris, but I think that SIL doesn't really want your family at these dinners. I really think that SIL wants to do the holiday with her family only.
You are the one obviously in the right here, and trying to maintain family harmony. Most people married to your brother would be delighted to have you for a SIL/great friend. You are an exceptionally good person, worthy of better treatment than you and your family have received from SIL.
I believe you when you say that she has a great friendship with DD. I have known people, though, who go out of their way to charm others when it is to their own benefit. I can see SIL wanting DD to love her dear aunt, to enhance her importance in the family.
A holiday meal is really a big deal to many people, a wonderful feast of things that we don't eat day to day, and luscious desserts. Your SIL's behavior is unconscionable. I can't imagine watching everyone else eat a holiday dinner, and basically being able to gnaw on the bones only. [Okay, I'm exaggerating here.] I feel so sorry for DH and DD having to watch others eat delicious stuff, and going hungry.
This is especially egregious when one considers how easy it is to cook without gluten. It would be so simple for SIL to accommodate your husband and daughter. She just doesn't want to make the effort. Anyone with a gram of decency could cook an entire GF holiday dinner, and it would be so delicious that no one would even notice the difference.
I'm in agreement with the PP who say that you should have your own dinner at home, and then go to SIL's. You could take a GF dessert if you want, but it would be nice if your family had dessert at home, too, so that you are not dependent on SIL for anything. Right now she is basically pulling your strings, as held by your brother.
Don't play her games anymore, don't give her any power over you. Disengage quietly so as not to cause any family drama. If she tries at some point to make it all up it will be a pleasant surprise.