It sounds like DS encouraged them to get this house, knowing it was a good investment.
As he was only 22 and new into finance (and not an estate agent), I would assume that OP and get DH would have thought long and hard about it and got a professionals advice too.
OP has said that it was a good investment because when they sell they’ll have money left over.
Yes DS lent his parents the money but if things had run smoothly then this wouldn’t have been an issue.
DS should have known that his parents were not good with money but luckily he did encourage them to buy this home with equity, so when they downsize they will have a bit of a cushion.
His treatment of OP is vile but they have a track record with being bad with money and are now struggling to make ends meet.
So he wants to keep track of incoming and outgoings and what jobs OP is applying for.
What’s he’s doing is a good thing but the way he’s going about it is wrong.
I think OP is probably taking it more personal because shes feeling out of control.
The loan or buying the house is irrelevant really because that’s done now.
What is relevant is how the loan is going to be paid back and what the agreement was regarding paying the board now that DS is paying the loan too.