Oh my goodness! I was in almost exactly the same position about two years ago. My dad was getting married again after seeing someone before he and my mom got a divorce. I did decide to go to the wedding and it wasn't that bad- at first. I wasn't sure how I felt about it all- I mostly was worried about my mom's feelings. However, my dad has perfected the crying at the perfect moments technique, so I went. Anyway, once I got there, I found out that I had to be a bridesmaid for my new stepmom! And then she gave me her bouquet to take home with me to my MOM's house! :wacko: It was pretty horrible all around. I don't mean to discourage you, but you absolutely have to know what YOU want without worrying about everyone else. I'm actually glad that I did go. I got to meet a lot of my stepmother's family, and be a part of their new world. At least you dad is trying to make you a part of his new life. It is really, really hard but you have to try to separate what he did to your mom from his relationship with you.
I definly agree with one of the earlier posts that said if you decided to do it, then talk to your mom and sister FIRST. My mom pretty much hates my dad now, but now understands that I need to communicate with and even encourages me to talk with him when I don't want to. Go to the wedding [/i] it feels right for you.