You don't have to tell your in-laws, or anyone else about your Asperger diagnosis. Just tell them that you find eating out to be very stressful, so you won't be spending your holidays that way any more, because holidays are meant to be enjoyed.
I suggest that from now on, you and your spouse prepare holiday meals at home the day or the weekend before, and then have the in-laws over in a more peaceful, and relaxed environment. With the food already prepared, you can just reheat and eat it, or serve cold meals, like sandwiches. You can even use paper plates and disposable cups on some occasions to make clean ups quicker and easier! Have the kids help with the food prep and clean ups, too.
We Aspies need to keep stress at a minimum. Take control of your holidays, so you can enjoy them. And, enjoy your life.