She Feels Pretty
Baby's the sister who thinks about joining the Peace Corps. Lisa's the sister who says:
LISA: I should've brought those coral shoes.
Doesn't Lisa realize that there are starving children halfway across the world who would love to eat those shoes?
Baby and Lisa are typical sisters, although Baby sometimes uses her intelligence to insult her simpler sibling.
JAKE: Max, our Baby's gonna change the world.
[…] BABY: Lisa's gonna decorate it.
A Baby vs. Lisa battle of wits is unfair. Baby has an arsenal at her disposal. Lisa has a flip-flop.
Underneath it All
On the surface, Lisa is your typical comic relief character. She's ditzy, goofy, and a bad singer in an age before auto-tune. If you look beneath all that, though, you'll see a deeper character than Lisa leads you to believe.
Lisa knows that Baby's their father's favorite. Because of her jealousy, Lisa's happy when there's a rift in Baby and Jake's rock-solid relationship. She expresses this to Baby one night in the cabin.
LISA: Oh come on, you don't care about me. You wouldn't care if I humped the entire army, as long as they were on the right side of the Ho Chi Minh Trail. What you care about is that you're not Daddy's girl anymore. He listens when I talk now. You hate that.
There's a moment where we see Lisa attempting to take Baby's place. She talks about Vietnam and China with her father instead of talking about shoes and wigs with her mother.
However, everything Lisa does is in an attempt to get a boy. She wants to look pretty for Robbie, and since he fancies himself an intellectual—as anyone who reads Ayn Rand does—she tries to impress him with her smarts, too.
When Lisa discovers that Robbie's a scumbag, she learns a key lesson: you do you, girl. And the real Lisa turns out to be a kind-hearted big sister who wants to pass the same lesson onto Baby. She does it in a sweet scene before the talent show.
LISA: Baby, I'll do your hair. It would look pretty if… No. You're prettier your way. This way.
So we hope Lisa finds herself, and that this self doesn't want to be a singer. Because wow, she's awful.