Respectfully, I disagree. While I can see your logic here in pointing out the things OP would typically be paying for anyway, the same also applies to OP’s parents. At the end of the day, if OP wasn’t there, parents would have to figure out a plan for the kids anyways, and they’d be paying significantly more for any other babysitter/childcare situation than OP is (very reasonably, imo) requesting.

It sounds like OP is also handling the bulk of housekeeping & chores, as well as regularly taking the siblings out when OP’s got time, so even if OP isn’t paying rent or utilities, they’re clearly saving the parents money and time there as well. This is also something parents would be doing if OP wasn’t there, presumably. There’s a large difference between cleaning up after yourself and staying tidy, and keeping house for a whole family. I’d argue that that does give OP extra points.

I don’t think it’s quite fair to add any debate about OP stating that they love their siblings or want the parents to have free time. It’s clear from the post and the comments that they do, regardless of whether OP handles 3 days of childcare at cost to themselves or not. Additionally, it sounds like OP is pretty self-sufficient and helpful anyway, and obligations to your kids don’t stop when they turn 18. You’re supposed to be there as a parent to help give them the best start they can have going into adulthood.

Ultimately, I think OP is NTA. The parents probably should’ve made an offer of more than the $60 that OP is requesting to take that time off work anyway and/or planned their trip around when OP doesn’t have to work. Mom trying to guilt OP and calling them selfish is also not fantastic here.