It's wrong to play favorites with your kids.
Jill was upset because her presents were never as good as her brother Jack's.
Jill wants to talk to her parents about it.
Her parents point out that most of Jill's things were bought new but Jack's were mostly used.
Her parents admit to it and point out that Jack didn't turn out well, so they wanted to raise her differently.
Jill realizes that Jack didn't actually have more spent on him at all.
Jill is upset because it feels like she's being punished for Jack's actions.