So, we've talked about How to tell your guests they don't get a +1. We've gone over 10 blunt-but-loving ways to tell people they're not invited to your wedding. But what happens when a non-guest takes it a step further, and actually RSVPs to your wedding? You know, the wedding you never invited them to. What happens then?
First: don't dodge
The worst thing you can do when there's been a misunderstanding is propagate it by not being up-front in how you deal with it. Yes, it's going to suck, but you can't put it off. When you hear from Aunt Madge that she's coming to the wedding you didn't invite her to, you need to address the situation quickly and directly. Don't put it off, and don't use platitudes.
Second: stand your ground
Be genuine, but be firm. Here's an example of something you could say:
Oh, no! I'm so sorry: there's clearly been a miscommunication. Due to budget/venue size/whatever limitations, we're keeping the guestlist quite small. This means we just can't invite everyone we'd like, as much as we'd love to — we do not have room for you on the guestlist. I'm so terribly sorry that we can't extend an invitation, and even more sorry about this miscommunication. I hope you can understand."
You can also try genuinely expressing your surprise. Own up the awkwardness of the situation, while also standing firm: THIS IS NOT HAPPENING.
Oh my goodness, this is catching me off-guard! This is awkward and difficult, and I'm so sorry to have to say this, but we don't have space on our limited guestlist for all the family and friends we'd love to have there — we aren't able to invite you to the wedding. I'm so sorry — I feel just awful about this situation, and I wish there's something I could do, but the venue size/catering arrangements/whatever are fixed, and I'm afraid it's just not going to work. I hope you can understand.
Third: be gracious and appreciativeRemember that these people want to come to your wedding because, well, THEY PROBABLY LOVE YOU. Sure, sometimes there are weird family power dynamics at play that have nothing to do with you, but ultimately, these people are people who care about you — and who presumably you care about, too. (Otherwise, you'd just say "Fuck off, you weren't invited.")
Find as many ways as possible to be loving, appreciative, and gracious in your conversation. Repeating, "It means so much to both of us that you want to be at the wedding" and "It's so disappointing that this won't work out — I'm so glad you got in touch" can go a long way towards softening the blow of what boils down to, "You can't come."
For those of you who dealt with this kind of thing, how did you handle it? Copy 'n' paste responses are especially useful!