Oh, sweet pea. I'm so sorry to hear this, and I'm sorry you had to hear that vile stuff from someone you thought respected you.

I'll say this - people say a lot of dumb stuff sometimes, often without thinking, but this doesn't sound like an off the cuff remark. What a hateful thing to say. You ARE your dad's real family, and if you never talk about this with them you'll subconsciously start to resent him for loving her.

I would talk to her first, if you feel comfortable doing that. It shows a lot of maturity that you have the foresight to see this is potentially relationship ending, and just proves to me how secure you are in your dad's love for you.

Just tell her what you heard and if she has any explanation for it. Her reaction will tell you a lot - if she shows genuine remorse and is mortified not because you heard her but because she realizes that is an awful and ignorant way to think, AND you feel comfortable afterward, then I don't think there's any reason to tell your dad. If anything in her reaction rubs you the wrong way, you need to tell your father immediately. You're his best man - you can't be standing by at the wedding while he marries someone that you know for a fact doesn't share a fundamental value with him (you).

Again, I'm really sorry you had to hear that from her. It makes me really angry for you and based on your post, I hope you still feel very lucky to have your dad in your life. Best of luck.