Almost five years ago, I started doing a Simpsons spinoff which has Maggie being a 13yr old. Well, one of the personality traits the older Maggie has is she is resentful of Lisa for getting all the attention for being the smart, 'good-looking' one. It's too the point where Maggie doesn't want to be in the same room with Lisa and she's taken her frustrations out by working out and has become a sort of Tomboy.
This Sunday's episode features the baby Maggie being revealed to be smarter than Lisa and now Lisa is resentful of Maggie getting all the attention. And now Lisa's changing her personality and is jealous of Maggie and isn't too fond of her at the moment.