I think Jean’s plot line was searching for her sister, which she did, and was resolved. And her lashing out was because she (and her sister) were seen as monsters so Jean behaved like a monster. Jean lashed out because she had no one. She believed no one could ever see her or understand her. Peter proved to her otherwise.

Aunt May taught Peter to be the caring man that he is. Peter shared that with Jean, both directly as himself and through memory with Aunt May.

I know she’s not “a kid”, but that’s the idea. She’s a kid and she’s learning from her mistakes.