I’m going to disagree with the Bees here who think you’re being uptight or stressing over minor details. Your stepmother sounds overbearing. It also sounds as though she hasn’t quite grasped that now her stepson is married, she/her family will not always be the priority.
Your place here is to do this: set aside your personal feelings about your stepmom, the religious stuff, etc. None of that is really anything you can change. Focus instead on what you can fix: the holiday issue.
Sit down with your Darling Husband and figure out a good division of holiday time that will leave you feeling like your family gets equal time (or at least as much time as you want with them). Then, it will be your **DH’s job** to inform **his** stepmother that he and his wife will be available on X, Y, and Z dates this year, period. If she quibbles or quarrels, it is your **DH’s** job to explain to **his** stepmother that now he is married, he is also a part of your family, and thus it is important to him and to you as a couple to spend time with them as well.
And that’s where the conversation between Darling Husband and his stepmom should end. There should be no debate, no apologizing, no more justifying, arguing, defending or explaining. She knows the dates you’ll be available. Period.
More generally, while it’s really your DH’s place to run interference with her, you also need to learn to become comfortable saying NO to her. “No, we won’t be there in time to barbecue for all your guests. No, we won’t be spending Christmas dinner with you this year; it’s my family’s turn.” And so on and so forth.
You grew up in a different kind of family. She’s never going to be that kind of family. So, you need to learn new skills in order to deal with her — skills you never needed to develop with your own family. Setting boundaries, without justifying, arguing, defending or explaining them, is the KEY skill you need to work on.
Lastly, you say you feel bad “trashing” this woman to your husband. So, stop trashing her. But don’t feel bad about discussing the matter of boundaries, calmly, with your husband, and figuring out how to present a united front on these matters.