Yeah, that's how I read it too (g/f called ex-wife). But really, this isn't about the dad OR his new girlfriend, it's about the original poster and her fiance. So I think it's really rude of her dad to make this situation happen after barely being divorced for 6 months. I know that people break up and move on, but be tactful about it and don't force acceptance. It will happen when people are ready, and a wedding vacation isn't really the best time for that.
I think you should just call your dad and be truthful about it. That you are happy that he's happy, but it's really not the best time to start "family" vacations with the new g/f. Hopefully, he will have enough respect for you and your FI (and possibly your mom!) that he will realize that it's just too stressful a situation. If he doesn't, you will have to weight the stress of having him and his g/f there against the stress of getting married without your father there. I am really sorry you are in such a sucky situation and I really hope that there is a good solution for you! Let us know how it goes if you talk to him about it.