POD to the other posters.
I also agree that both sets are taking advantage of you. If your father asks you to buy $50 a week of lottery tickets when he's with you, and buys a lot less when he's paying himself, yeah, that's a problem.
A couple of things to think about
- you want to train them to reasonable responses *before* you retire. Because the expectations will probably get worse when you do - you're not working, so of course you can come down whenever they want. And they've already got you trained to pay for stuff, so the expenses won't go down.
- The only way to stop this is to say NO. The next time your SIL calls and asks you to come down you can say "Oh, I'm sorry, I don't have any vacation left" or "Oh, it's really busy at work and I can't make it" or simply "No, I can't do it this time."
You're not actually causing them a hardship. They could easily pay for a pet sitter, and there's got to be someone who can look in on your dad once a day, or be in phone contact, or pick up something from the store for him.
But what it comes down to is that they are taking advantage of you because you let them. They've got a pretty sweet deal going, so they're not going to give it up voluntarily. So you have to say no, learn to make it stick, and weather the storm if you want it to stop.