Well, yesterday afternoon I sent a "blanket" email to all sets of parents... 1) Giving them an update on where we're at with RSVPs, and 2) addressing the fact that some people are asking why they weren't invited, and that we need to respond politely but firmly that we cannot accomodate everyone we would have liked. Then I invited our parents to have these people call me directly if they're uncomfortable letting them know.
This afternoon, FSMIL called me back and I pretty much told her the same thing. She apologized and told me she would call the step cousin's half sister and explain that we cannot accomodate. She claims she didn't understand that it's a "by invitation only" event... which may or may not be true. But once I explained how much FI and I are paying per person for this reception and described why it's important that we keep a handle on the guest list, at least she didn't argue with me.
It could have been much, much worse. Hopefully this won't be happening again. Lol.