I don't believe in having anyone at my wedding I don't want to be there!. It's your day and if your boss is a mean/nasty person, they know it and should not be surprised. Heck they may not want to go anyway (unless it's a woman in which case she may want to go so she can spy), but I don't think she can do anything about you not inviting her. Do you owe her one though? You could just say what I say to people who aren't invited, it's an intimate affair, and your boss may speculate, but has no proof how close you and you co-workers are.