@ASDFGHJKL she's allowed to be jealous and resentful, afterall she did get mistreated by her parent. When has jealousy ever been rational? Aslong as she did not take it out on her brother negatively I don't see her being sad as an issue because she's allowed to feel those emotions. You're also ignoring the fact that stuff that happens when kids are little are more likely to affect someone when they get older.
