If she had given OP a choice instead of 'voluntelling' them to babysit her kids maybe I could get your POV. But she made the choice to have kids, not OP, and OP told her they weren't comfortable with the idea. The kids are not OP's responsibility family or not. Some people don't like kids and that's okay. Doesn't mean they hate them, doesn't mean they're evil, but everyone makes choices and OP's was to not deal with kids they didn't have to deal with.
Would it have been nice for OP to help out? Sure. But OP doesn't have to bend over backwards for someone just because they're family. Besides, they didn't get to enjoy their trip anyway because the sister dumped the kids on them in the end regardless of the upgraded ticket.