I don't know why, but her response kind of bugs me. Maybe because it seems so flippant, like it is a joke?
Uh, yeah. "Surely it wasn't one of my little angels - must have been a poltergeist!"
I know a lot has been said and I'm commentig on something pretty far back...and I also am not picking on this particular post. It's one of a few I could have quoted.
But, I do think it's kind of unfair to get on the SIL for assuming it wasn't one of her kids. Sure, her response was kind of odd, but it seems to me she was trying to make light of "mystery." Just like the OP, she probably thinks it was a kid that doesn't belong to her and doesn't want to come out and accuse them directly.
It would have been better had she responded with something like "Yes, I heard the story in triplicate and if I get any useful information from the kids I'll pass it on", but outside of that, I don't think it's all that unusual for her to jump to the assumption that one of her kids wouldn't have done something like that. It's the assumption the OP made about her own children and without any concrete evidence of who did it, maybe it is easier to try to laugh it off and forget about it.
OP, I think you're fine for going through with the sleepover and the measures that you are taking seem fair to all the kids across the board. Hopefully, this is an isolated incident.