You do come up with brilliant out of the box ideas, so I'm on the fence with this one. The other thing is I don't think any of the children would have done the stuff that had happened. They might play tricksters sometimes, but they wouldn't mess with your house (hopefully).
Count me as someone else who finds her reply odd. At least she didn't get angry... ? The part I don't like is what I've left above. She says she thought you
were playing a trick on the kids, okay, fair enough I guess, but then when you tell her it wasn't
you, she's like "I'm on the fence with this one"? Does she still seriously think you might have done it, and are just extending the joke to her? And then she goes on to say she doesn't think it could be any of the kids... so who would that leave? I was assuming her "ghost" comments were joking, but if she doesn't think it was the kids or
you... what's her idea?
I think I would reply to let her know that indeed, it happened, I don't know who did it, and I'm upset about it. Also, I noticed in your email to her you didn't say you were suspending the sleepovers until this is resolved. If that's indeed what you've decided to do (which I personally think is a good idea), I would be sure to tell her that in your reply. Maybe that would make her take it a bit more seriously, and finally get her to realize you're not just playing some prank on everyone. PP have suggested good phrasing for that--I think one was, "We won't be doing anymore sleepovers for the time being, as the kids can't seem to behave when they're together." That doesn't point out any one kid, or her kids, as those at fault. From the timeline of events you posted earlier, I can see why you thought it was the nephew, though!
Maybe I'm being silly with this, but she talked about ghosts a lot in her email, and I think you said the 6-year-old just came up with this ghost fear suddenly. It really makes me wonder if they've been talking about ghosts at her house a lot lately for some reason, and that gave someone (nephew?) the idea to prank your son with a ghost story and some "ghost" evidence. Maybe he talked to your son about ghosts, then was going to produce the evidence but your son fell asleep first. Red marker scribblings on the countertop could be someone's simulation of "writing in blood," mwhahahahaha...
Some of the other stuff, I still wonder if one of the pets could have done it. Obviously it was one of the pets who threw up; that might have had nothing to do with anything else, except someone made a lame attempt to "clean it up" with the soap. Could one of the pets have gotten into the pistachios and found they didn't agree with them?