S.S.
K., I was like a stepmom for two years until my boyfreind cheated on me and we broke up. But, his kids hated their mom so it wasn't hard for me to win their hearts. If your husbands kids are happy with their mother and their traditions, then trying to but in and be a part of it may cause more problems than you think. Create new traditions that are nothing like the traditions they already know. Be different than the mom. Be a refreshing change for them. Also, respect their love of their mother and their old life. If you do, they will have more respect for you. They are at a tough age for this. Ask them openly, 'what are some things you guys have been wanting to do that you dont get to do?' Let them answer you, no matter how long it takes and make sure they know that your ears are always open. Do those things with them. For the girl, it may mean coming down to her level of a 15 year old and take her shopping at the mall. Buy her the outfit her mom would say no to, you can argue w/ mom later. But, at least she saw that you understood her feelings and stuck up for her.
As for the Christmas traditions- learn about the traditions they already have. Ask questions about them and act interested, but don't try to be a part of them . There is no room for you, hate to say it, and you have to admit that. Then, create new traditions for the kids that are yours/theirs alone. Completely different and new. Always make it look like you have an open mind, never act hurt or intimidated. Have the attitude, 'if you don't like this, fine. I'm still gonna enjoy myself.' -They will come to appreciate you.
Your hubby will also be impressed if you create a different 'home' than what they have w/mom. And I realy stress the word different.
Hope this helps, if you have more questions feel free to reply.
S. S