Thanks, everyone. My mom's reaction left me with the impression she thought I was being a brat by not wanting to cram into the house. Instead of emailing Aunt Diana I wound up just calling her, and saying, "Hey, DH and I decided to get a room at the (hotel), they had a great rate. What time is dinner on Christmas Eve?" It didn't even phase her, and we chatted about what I'm getting the girls for holiday presents, as they celebrate both Christmas and Hanukkah at their house.
The more I think about it, the weirder my mom's behavior is - Diana was very insistent that "everyone come to my place" for Christmas this year, and staying together is just always how my family has done things. My grandma and parents live in my hometown, about 2 hours away from Diana and family, and four hours from DH and myself. Usually that's where we do Christmas, and Diana & Co stay with Grandma while DH and I stay with my parents. So it's not like we all MUST stay in the same place, right? I think what irked me was that Diana insisted we all MUST come to her town this year when it's not as easy, but that's not really the point. DH and I have a choice to go or not, and while I want to be there, it'll be nice to have our own space.