Well, my fiance recently just got a text today from my sister saying how ungrateful we are because they helped us out with giving us 14k and how we couldn't let them sleep in our place for one night. It brought her to the point of tears because it's making her, and I, feel like bad people.
This is the point I would have gone to the bank, withdrew the $14k, drove to wherever the parents were, and gave them the cash. They'll try to refuse it, just drop the wad of cash and run out.
Next time they ask you to drive them to/from the airport, tell them to hire someone with that $14k. Ask to crash at your place, tell them to find a motel with that $14k. If you're feeling particularly generous, you can even find a cheap-ish airport shuttle, motel that accommodates pets, etc.
They'll give you the cold shoulder for a while. Probably talk bad about you (which'll make no sense...how can you be bad for accepting the $14k gift, and also bad for returning it?). Might feel that your fiancee is the one causing these issues, since you used to be such a good boy (even if they don't think it, it'd definitely get sympathy from others). Over time, they may realize that guilt-trips don't work on you anymore, and the relationship may be able to progress to something better. Or, they may really ramp up their efforts (if a little bit of guilt didn't work, let's try a mother-fudging TON of it!). Could have to end up cutting off all ties.
Guess I'm saying...I'm not that great with family relationships. If someone wronged me and brought my wife to tears, and demanded that "I" apologize, well, I'm not going to have anything to do with them anymore. If someone is genuinely nice and kind toward me, I'll do anything I can to help. I have a pretty good relationship with one side of my family, and no relationship with the other side (the person who's part of both sides, it's a little awkward). So, perhaps you could do something similar, but worded much, much, much better than I would.
I like the idea of paying for whatever they need. Keep all receipts. When they ask you to drive them to/from the airport, hire a car service and keep the receipts. Need a place to crash, just book a motel room for them and give them the details (if they then refuse...say "but, you needed a place to stay...it was urgent...I found a place...wtf??"). Feel free to explain that you're using the money they gifted you (I dunno the best way to approach that, or to word that, but something like "Well you helped me out so much with that $14k, I thought it'd be rude to not use that money to help you out when you needed it"). Once your receipts hit $13k, the next request should be met with a "sure, but just so you know there's only $1k left out of that money you gave me." Once it's all gone, feel free to keep helping out, but I'd just want to make absolutely sure that they knew that what they had "gifted" you had been used up in its entirety, and they couldn't hold it over your head anymore. If they even thought of saying "but that time we gave you $14k" you can whip out all the receipts and say "yeah, but that time I paid to get you to the airport, that time I paid someone to pick up your medications, that time I paid someone to take your dog to get their hair cut, that time I paid for a motel room, that time I paid for...you know, this is silly, let's talk about something else. How about that weather?"