DH has a relative married to a woman who has very unusual self-imposed dietary restrictions, but who never fails to insist that we eat out when we visit. Only trouble is, DH & I have discovered that the only places that serve what she will eat are restaurants with what we consider horrendously bad food, and are ridiculously overpriced. In defense, we started eating ahead of time. So, we go out on our own early, eat what we like, and then meet them at their restaurant of choice, where we have drinks and maybe some bread or such while they eat a full meal, and everyone is happier.
Perhaps you might adopt that strategy? Scope out places where you can eat early, get good food that works for you, and then go to the family party with a full stomach with the object of spending your time socializing instead of eating. You can even tell them this; that you know that navigating your limitations is difficult with a large party, so you've decided that your gift to them is eliminating the worry of having to deal with it. Then you can drink the cost of a dinner's worth of fabulous cocktails instead!
