fiance and I have a deal that we must be together to open RSVPs. It's SUPER exciting! Congrats!
I know what you mean. We got one rsvp back from my FI's uncle and aunt, there was no return address, and just marked as Mr. Smith was the only one coming. The problem being, we had a few single Mr. Smiths we sent to. Thankfully when we sent out the invites we numbered the response cards on the back, and so it was quick enough to figure out who it was from.
We also had an invite RSVP 4 (them, plus 2 kids) when it was just for them. The husband we spoke to before the RSVP came back and he asked if children were invited, and we had told him we were keeping it to family only. He said that's fine, and would make sure wife knew. Got the rsvp back, marked 4, then about a week later got a text from the husband saying to count them in for the wedding and the wife knows it's a 'no kids' event. Not quite sure if we should say something, or just suck it up and chance having an extra 2 kids there.
You could always call/email/send a quick note (or ask your parents to if it's their friends) saying "FI and are excited that you and Mr./Mrs. are able to attend the wedding. We look forward to seeing the two of you at the wedding" and see what you say. A little effort on your part, but worth it to know who's coming.