We don't hear much from Finch's little sister, who's only eight, but we know she's a troubled little girl. (Sadly, that seems to run in the family.)
How can we tell? Well, she gets in trouble at school and cusses at the dinner table. And in her most memorable scene, Finch walks in on her cutting all the "mean words" (21.70) out of books. She says:
"They shouldn't be there mixed in with the good. They like to trick you." (21.69)
Is she just being childish? Or is she showing signs of the bipolar disorder that her father and brother both have?