Mom decides to bring teenage daughter to adults-only weekend trip without telling anyone, then demands sister pay for daughter’s stay: ‘[She] doesn't take up much space’

Are we responsible for other people's children?

The short answer is: no. Parents alone are responsible for their children, and that involves education, time, recreation, health and money. Yet, sometimes parents seem to forget that not everybody out there has children, and that no one but them is responsible for those kids, even if they are a bit older. The rest of the people around them can be a big help, especially with younger kids. But even when they do it gladly, they don't replace the parents, nor should they take responsibility for them. 

That can be a source of friction with people, especially between family. Since we're all related, the boundaries between what relatives do out of the goodness of their hearts and what the parents see as an obligation for them can be a bit blurry. Aunts and uncles might be expected to act as substitute parents, or accept their sibling's children as their own. While the bond between nieces and nephews and their aunts and uncles can be beautiful and very close, those aunts and uncles still have lives of their own, outside of those children that are not theirs. They can give gifts, watch them from time to time when their parents have their hands full, be there to support them, take them out, and such, but they are not the parents. They can offer to give as much as they want, but the parents should never expect their siblings to take charge of their kids.

A woman on r/AITH shared that her sister doesn't always understand boundaries. She had organized a weekend trip for both of them, plus two close friends, and split the cost of the stay between the four. On the day of the trip, the sister showed up with a fifth person, her sixteen-year-old daughter. The girl ate most of the food they had bought, and the women spent the whole trip revolving around the girl's schedule. Her mother never told any of them she was planning on bringing her, and just assumed it would be fine. Her sister disagreed and asked her to pay the equivalent for the girl's stay, since she was a fifth person not meant to be there in the first place. Now, the girl and her mother seem to be offended by the aunt and are telling people that "she only cares about money."

What do you think? Was it alright for the aunt to ask for the money for her niece's stay?