If you are planning on becoming a part of her family, then you are going to have to (at the very least) learn how to interact with her son's father on a regular basis. The ideal would be that you could at least if not be friendly, at least tolerate him, because he will be a part of your life, though your step-son. There are lots of family orientated holidays throughout the year, it's going to make things very uncomfortable for all involved, if you can't stand to be in the same room, or if you are constantly making your wife choose between spending a holiday with her son (if he wants to spend it with his father), or spending it with you.

Personally, I would spend Christmas with my fianc, if he invited me. All that matters is that you are part of her life, and that makes you part of her family as well. You need to find ways that you will interact with her son's dad, so as to make you feel more comfortable. I would advice you to just take it as a way of getting to know her whole family, and be positive about it.

Hell to the no!

Yes you are getting married to her so the kids dad will be at these things, just be cool and act normal. No need to get all excited about it. I have two kids and christmas is almost pointless without them don't get side tracked and make the best you can out of the situation.