It's not the first time that I've read about a family vacation going wrong. It's something normal, and it tends to happen more often than not, and it's for one reason only: expectations. When we're kids, we spend vacations with our parents and have a certain idea of what going on a trip means and what things tend to happen, and which ones don't. However, as we grow up, this can change and vary.
And the situation with these two sisters is a great example of this. The story posted in r/familydrama explains that she's a teacher and that going on this trip means a lot financially, and that she's been saving up so she can afford it. Her sister, on the other hand, is a lawyer and can easily fund this whole thing for her husband and two kids also.
But the problem is not really the money, rather that the expections a previuosly mention.
The successful sister is expecting that the teacher is going to be able to babysit her kids during the whole vacation for free, so she can enjoy "adult" time with her husband. She's thinking her sister is not going to have any trouble, considering she's child-free and she loves them… but this has nothing to do with love. It's about everyone enjoying the vacation and disconnecting completely from the routine. Unfortunately, babysitting is not part of the plan.
What the Original Poster is proposing is that the sister should cover some of the costs of her trip if she's expecting her to take care of her two kids… which, to me, sounds like a fair arrangement. Although it's not what she would want (since she doesn't want to babysit at all), it could be helpful for her to get some help with money, and instead of paying her back, she can babysit both kids, and the problem is solved.