Is her mom really asking a lot of her? Or the bare minimum? 

There is no shortage of older siblings who are tasked with the near-constant care of their younger siblings. This is a familiar dynamic in families where both parents work and don’t have a grandparent, aunt, or uncle who lives nearby and is willing to share the babysitting load. Just because many families do it doesn’t mean it’s right. There’s a significant difference between an adult child moving back in with their parents to help care for their younger kids out of genuine desire and doing so because they feel they’ll be exiled from the family otherwise.  Young adulthood should not be about finishing the family your parents started. Young adults need that time to find themselves and build the life they want; otherwise, they’ll end up resenting their parents and feeling stuck in that teenage stage.

There is a big difference between your parents forcing you to babysit for hours at a time every night of the week and asking you to watch the baby for 30 minutes. A lot of teenagers don’t want to do either of those things, but it’s completely reasonable for a parent to expect their kid to help them with some babysitting every once in a while, especially when it’s for something as crucial as showering. The teen in this story is resentful towards her mom for continuing to have more kids, so she’s refusing to watch her baby half-sister while her mom is in the shower.