I think the fact that you can still see the original reservation in MDE is a positive. If they were really out to scam you I would have expected that to have disappeared PDQ after getting the money for the last night.
So it may genuinely be that their son is very sick. But that doesn’t help you.
When did you last hear from them? Maybe email them again and add a “reply received” to it (the one that sends you notification that they’ve read it).
Other things you can do, which I’m sure you have because you’ve said you have rented before. But maybe helpful to others.
Where did you get their details from? Check their status, post count and so-on. Do they seem legit? Any hints on the forums there that there’s an issue, have they posted anything referring to their son being sick? Ultimately you could contact the Admin and let them know there’s a problem. Not that they can necessarily help but they can take action for the future.
For more peace of mind you could try googling them and see if you can get a phone no. If so, does this match the area that is listed on the deeds?
Otherwise I guess you can contact your bank or however you paid them and see if there’s any way to dispute the transaction, if it comes to that.
I really hope it works out OK. Keep us posted.