Maybe your mom and step dad will remain friends and then the holiday thing won't be an issue. it is not unheard of. I have a friend who's family is like that.
As far as how you wll look at your own marriage, well that is what it is "Your own Marriage". I have been with the same man for 20 years and can not imagine being with anyone else! And my mom has been married 4 times so don't think the thought has not crossed my mind. It is what you make it. And besides look how long they did stay together. If this is not to personal...why are they breaking up?