I agree with Job. I wouldn’t let the kids go on their own either. I’m also a little concerned by S use of the word “poisoned”.
It’s a strange word to come out with and sounds like he’s heard the term too many times...maybe from your H?
I think your kids have been through the mill enough without hearing negative opinions of you from your MIL, especially if they stayed there and FIL was tanked up after a few drinks.
I would do as job suggests and stay in a nearby hotel. Then S can never accuse you of preventing him from seeing his GP’s.
It’s one thing after another isn’t it. My thoughts are with you Gerda....I don’t know how you haven’t cracked up by now.