I grew up with family that helped each other out, but on a much, much smaller scale. We didn’t have a lot of money when I was growing up, so I can relate to that part.
In my family, if someone couldn’t live independently for one reason or other (sometimes health-related), they lived with someone else. They didn’t get to live in a house paid for by others. (I know you said they pay the mortgage/utilities, etc. But your wife still owns it and if they became unable to pay, it would fall back on her.) Paying for a broken boiler is your wife’s responsibility, as they aren’t the owners, but effectively just tenants.
My grandfather subsidized his mother and step-father’s weekly groceries, but they never knew it. They couldn’t get out for grocery shopping, so he’d bring them what groceries they asked for. When they asked how much it was, he always under-quoted the real cost. They wouldn’t have stood for it if they had known he was paying for even a small part of their food.
New cars? No, they’d get someone else’s old car for free when that person decided to get a new car for themselves. I’m concerned about your wording, “so DW gave in”. Is she being pressured by them to keep buying them things? Wouldn’t used cars have been sufficient? For that matter, how about one car? If they don’t work, do they each need their own car, when they can’t afford to buy it themselves?
Everyone paid their own phone bills. Adding your in-laws to your group plan is fine, but can’t they afford $20 per month? We’ve always used prepaid phone plans and it’s never cost us $20 per month. I just bought a new phone last month - $119 plus tax.
A monthly gym membership is a luxury, not a necessity, IMO, especially if someone else is footing the bill. They can’t afford to pay for their phone and the monthly cost, but they can afford to feed and care for a dog, except for the vet bills. I like animals, but they are also luxuries when you can’t afford to pay for your own basic expenses.
How often are you footing the entire restaurant bill? Do you and your wife ever go out by yourselves? Family vacations? Growing up, many of my relatives had enough just trying to manage putting food on the table and keeping a roof (usually rentals) over their heads. I heard about vacations from my friends, whose families could afford to take them (or maybe not, maybe they went into debt). But it was a luxury I didn’t get to experience until adulthood when I could afford to pay for my own. Do you and your wife ever go on a vacation without them?
The problem with you paying for all this stuff for them, many of which are luxuries and not necessities, is that now they’ve come to expect it. And they’re getting nicer stuff than you are! Why are you driving 10 year old cars when they get new ones? Maybe worse, is that they’re not learning to manage money. They want something, you or your wife pay for it. You’re experiencing the consequences, not them. You’re putting their welfare ahead of your own. And it sounds like you’re starting to resent it, and understandably so.
If you’re still inclined to help, it’d almost be better to cut them an annual check that they have to learn to budget with. Then they can’t keep running to your wife asking her to keep buying them stuff. You can live your own lives and so can they. You can get together when you want to, but are not obligated to.