I can't tell you how much this story resonates with me. In my family, we're eight cousins: four are older, and the other four came 12 years later. That means that my fellow older batch and I were ALWAYS assigned the babysitting duty when it came to big family events, dinners, Christmas, etc. At first, it was kind of okay… At least we could all enjoy some time with them, and it wasn't that big of a deal.
But time went by, and we all grew up. That meant that my cousins and I (the older ones) wanted to sit at the grown-up table to chat with them… it seemed more normal, and it was what we wanted. During one Christmas, my grandmother was arranging the tables to have us sit with the four littles, and my oldest cousin told her that it wasn't to happen that year. She was confused, since she thought it was "an unspoken rule" of our family. Although during that dinner we ended up sitting with the littles, someone stuck with my grandma, and for New Year's we were at the other table, and we were happy about it.
You can imagine how I felt when I read that Reddit story. It was like listening to myself from a couple of years ago. But the worst part is that she was far away from her older relatives, and she had to text them to come and pick their kids up… I think she did okay in leaving. It seems that the only reason that she was invited was to babysit the little kids.
There's no problem with watching them for a bit, or even agreeing to babysit for a while. But it has to be talked about properly, and not assume that a vague question is all it takes. Also, I found it funny how the family got mad when she left because now they have to take care of their kids during dinner… that's just not how things work.