OP the problem here is firstly that you’ve cast some sort of Disney-like view of your “lovely lovely BIL”. He was an innocent 29yo virgin at a young farmers party (why the fuck is a nearly 30yo at a young farmers party, a bit creepy and how do you know he’s a virgin?!). She, the evil witch, turned up uninvited (I used to go to the YF parties didn’t realise you needed and invite), and stole his virtue - he of course as a grown man had no say in this and was a little innocent lamb let by the evil witch who was much younger than him.
Fast forward several years and the lovely lovely innocent lamb is clearly earning an absolute fortune, his DW is a SAHM, he’s barely at home and his kids are in private school. Again being an naive doe eyed lamb (but also intelligent enough to be earning an absolute packet overseas) had no idea about finances and couldn’t take the time to have some control over the cost of a kitchen.
You for quite sexist reasons seem to intensely dislike her, and RELISHED the opportunity to be able to control her life in some way. I think the only reason you gave out the loan, despite the face that your BIL can afford the repayments himself, was so you could control her. Money well spent or what. You can’t impose controlling terms on a loan. And I doubt you could sue her for not getting a job, seeing as the loan is being repaid.
Bottom line is you shouldn’t have lent them the money. So the kids would’ve had to come out of private school - so what. Plenty of kids “settle” for state school and they’ve had most of their years in private education so have been given a good start. You should have said “no sacrifice the school fees”. If he said they were destitute and gonna lose their house then fair enough but they just want the exact same lifestyle with no sacrifices, and that’s not how managing debt should work.
I also think it’s total bollocks that your friend sent you a screenshot in a “Let’s go to this destination” way. It was being bitchy. Because you’re seemingly obsessed with this woman and have probably told all and sundry about the loan and your mate couldn’t wait to grass on her
As for the paternity of their DD - what does your BIL have to say about this?? It would be very odd of him to have shrugged and said “meh”
How is your loan legally binding? I mean it surely can’t be THAT big a loan if they can still send their kids to private school!