Welcome back ladies and gentlemen of the internet. There's a lot of heat and contention around free babysitting. When you're related to someone, there's a certain level of expectation that they'll help out with things for you. Family supports one another, that's just ingrained in being a family. But being in a family also carries its own dynamics and need for boundaries. You can go out on a limb and help a family member, but if it's a constant thing, or something they could easily handle alone, it may be time to put up your arms and tell them the truth.
This brings us to the Reddit thread. In this story, the original poster (OP) is a woman who planned her whole vacation well in advance. She was looking forward to it immensely, until her sister came to her demanding she forfeit the vacation to babysit her children while she and her husband take a spontaneous trip. The OP is clearly in the right, but what's shocking is that her family calls her selfish. You'll have to read the full story for all the juicy details.