muffin the thing about your OP that disturbs me is the history of them lending you money at credit card rates in the past. I can’t imagine lending my adult children money and getting them to pay greater interest rates than I would get from a bank. Just for the record, the only loan we have made was to DS to fund a postgraduate degree, and that was interest free.
Like your PiL’s, we are comfortably off and live well within our means.
We do give gifts to our DS and DDs, but they all get exactly the same at exactly the same time. What they do with it is entirely up to them. They can put it into DGC’s college funds, they can buy furniture, they can put it in their own pension funds, or they can go on holiday. I don’t care what they do because I know they are all responsible with money.
I enjoy passing money on to them now, and feel very privileged to be able to do so, rather than waiting till I’m dead
In your position, I would ascertain that this is indeed a gift and not a loan.
If it’s a gift, accept it with a good grace.
I understand your feelings of wariness, and I don’t think it would be churlish to refuse, but I do think you’d be cutting off your nose to spite your face.
Hope that helps.