LJ...
hmmmmm It really does sound like an uncomfortable situation, for you anyway. I like hearing of your supportive efforts in the past to include these important people in your lives. I think you have been open and accepting. Obviously, you are not ready for the full blown, "we're family now" thing, and I can't blame you. Especially at such a special time of year, the holidays, you should not be in such an awkward predicament. I think you need to be fully upfront with your husband and tell him how you feel. Perhaps you can compromise by agreeing to a short visit, but not the day/dinner thing. It certainly sounds like he has a close relationship with these people, but is he holding on to too much of the past? He has married you, after all. Maybe it is time to start new traditions together.
I know it is easier said than done, and every situation is different. I still have a relationship with my Exhusband's family, though we do have two children together, too. His family is wonderful and has always been so kind to me, my family and to my new husband. And there are functions throughout the year that involve the kids and we all participate easily. Still, I don't think I would consider sharing the holiday all together. We have separate lives now and the line has got to be drawn somewhere, right?
Kind of reminds me a bit of my Mother in Law. She kind of "forces" her friends at me. I explain to her that I need my own time to develop feelings and a relationship with someone, and it isn't dependent on whether or not she thinks they are wonderful. I think this applies in your situation, too.
Good Luck. Let me know how you make out. SGE