Dicey’s younger sister Maybeth is not your average student. She struggles to understand math and reading, but somehow excels—and we mean excels—at the piano. She's a straight up prodigy.
And thank goodness she can play, too. Seeing her find herself in music helps assure Dicey that Maybeth won’t go mad like her mom. It’s a definite worry: "Dicey thought she knew what he was thinking—that Maybeth was like Momma. ‘Nothing’s wrong with her,’ she said quickly. ‘You know that and I know that, James’" (5.20). See, Dicey and James are worried that Maybeth's learning issues stem from her mommy-issues. But her piano skills help calm their nerves.
Not only does learning to play the piano suggest to both Maybeth and Dicey that Maybeth’s neither stupid nor insane, it teaches Maybeth her own lesson in reaching out. For one thing, she accepts Mr. Lingerle's offer of piano lessons, which is kind of a big deal for the whole family. Dicey has to accept it too, even though her response when Gram suggests "special lessons" is decidedly defensive:
"Take it easy, girl," Gram said. Her eyes were laughing at Dicey. "The kind of lessons he was talking about were special lessons […] Lessons for someone who is talented." (2.107-109)
When Dicey sees that Maybeth has a special talent for music, she learns to trust that Maybeth may just turn out fine—that there might be a place for her in the world that isn’t an asylum. It’s all part of Dicey’s lesson in seeing her siblings as individuals who will eventually be capable of taking care of themselves.