MIL and SIL are the prime suspects for supplying it (he's not physically able to go get his beer himself) though neither will admit to who's doing it and both are trying to justify it.
Cunning, baffling and powerful. Also insane.
Too much of a hardass? Are they your boundaries? Are you comfortable with them? Will you enforce them?
When my children are involved, it's easier to place and hold firm, healthy boundaries.
As far as the financial situation goes, if repaying their kindness is the right thing to do, it's not your responsibility what they do with the money. I'd see withholding fair repayment as controlling.