What do I do about an univited guest at my wedding?

well she'll figure out she's not invited when she doesn't get an invitation. but you should come clean with her so she doesn't spend any money on airfare anytime soon, and tell her you can't afford to have a ton of people, you're keeping it very small and intimate, it's family only, or the venue just doesn't fit more than X number of guests. it's very rude for her to assume she's invited, I never assume I'm invited even to very close friends' weddings. people are watching their money and downsizing these events, so it's totally reasonable for you to tell her it's a very small affair.