I get that you are worried about your inlaws being taken advantage of, but there might be things they know that you do not.
I had a similar issue, but to a much lesser extent, with my SIL. She was divorced, single mum of 3, and she relied on them so much. Not to take them to school, or basic things like that, but FIL did everything around her house, paid for new fences, a new bathroom, a new kitchen (she said she would "clean their house" to pay the "debt" but did it twice then stopped), and it was awful to watch her just usurp all their available funds, time and goodwill.
We did have discussions about it when they brought it up, and complained, and we said the more they gave the more she would take. But she was in a bad place, and has since improved greatly, now works, has a new husband, the children are older, etc.
But the truth of the matter was that whilst FIL didnt like the arrangement, it was a very co-dependent thing between her and MIL - MIL needed to be needed. She still does.
So we didn't get too involved, but we wouldn't take too much of their nonsense, or hers, when either one started complaining about the arrangement. The amount of free child care, house improvements, food/meals, and general support she had from them was breathtaking.
She will probably give it all back and some when they are old and need care - she is that kind of person and that's something, it's worth more than a figure you can put on paper really, and the amount of support and care she gave her elderly grandmother, along with MIL, when she was housebound and dying probably makes up for most of it, so it is a very different situation to yours really.
But I do see your concern, and it's easy to say "none of your business" but it kind of is in a way, depending on how it affects the family unit as a whole, or your PIL's as people.