If I were you, I'd go because it is his day. But I would make sure to tell him after the fact, maybe not tomorrow, that you wished you could have been more a part of the plan making process because you're his daughter, too. That is such a tough situation, and while I've never had to go through a parent remarrying or having step/half brothers/sisters myself, I can imagine how hard is must be for you to feel like the third wheel to your own dad. I'm sorry.