Best thing you can do is to not think about it. To be quite honest, most people who are going to the con will likely not care much if someone they dislike is going - they'll be too focused on having a good time of their own. So don't worry about it, and don't try to cause a confrontation, and there likely won't be any trouble for anyone.
After all, while her costume may be obvious, that doesn't mean you'll be around her constantly. Cons are big places, after all! =^~.~^=
PezCat