Claudia has an older sister named Janine. While Claudia focuses on her art, the club's new playgroup, and the Baby-Sitters Club, Janine is constantly hovered over her computer, studying. Janine is perfect, takes college level classes, and gets all A's, even on the college level courses. Claudia feels like she can't compete with Janine. To her, there only one member of the Kishi family who understands: Mimi.
Claudia's grades begin to slip, so her parents tell her that she has to either give up art and babysitting, or get better grades. Mimi begins to tutor Claudia. Claudia likes it WAY better than when Janine tutors her.
When Mimi has a bad stroke, it is up to Claudia to improve her own grades, help Mimi, babysit, AND help out at the playgroup.