I think you're making the right choice. This is the best time to reestablish boundaries.
Absolutely. You're getting married soon, and establishing your own traditions. This is the natural time to make changes about how holidays are celebrated. Otherwise, things have a way of getting carved into stone, and it's much harder to make changes later.
You are not being at all selfish or unreasonable. Especially since you invited them to drop by and visit when their original plans involved being in the area. You set a clear boundary, but also extended an invitation.
Don't make the mistake I once did of bending over backwards to compromise and placate your family in regards to Xmas. Many years ago, after a few years of always spending Xmas with my mother and visiting my husband's family later in the week, we decided to go visit his family for Xmas. You would have thought I was cutting off all contact forever! My brother lived nearby, and was planning to spend the holiday with her. But I was leaving her "all alone."
To placate her, we came over on Xmas Eve, spent the afternoon and had dinner. Made the 3 1/2 hour drive to his parents, arriving at around 11:30 pm. Really not how I would have liked to do things, but I thought we'd get our obligations out of the way, and she'd be happy. And we'd have the whole next day to enjoy, right?
But when we got to his parents' house, his aunt and uncle and cousins insisted on opening all the presents right then. And it turned out they'd already had Xmas dinner without us. Even though MIL knew our plans, she never bothered to tell us she was going to do that. It slowly dawned on me the next day that not only was the whole present opening thing done, but there wasn't even any Xmas dinner to look forward to. We had to scavenge for leftovers in the kitchen. Odd.
And after all that, I was still in major trouble with my mother because I left her to "wake up alone on Xmas."