Is she supposed to know through osmosis that she’s supposed to pick up her niblings? 

Nobody likes to sit in the school pickup line. I remember my mom had to show up to school like an hour early to get a good spot in the school pickup line. If I were to do it all over again, I might’ve just taken the bus or walked from school so I wouldn’t have to put my poor mother through the slog of sitting in her car and waiting for me to get out. 

Some people are willing to go through the school pickup ordeal for their children’s comfort, or because they don’t have any other options. The mom in this story is more than happy to let her sister pick up her kids from school, and she doesn’t even think she needs to ask if her sister is free to do so. She just expects it at this point.