The Disappearance of Sister Perfect — Jill Pinkwater

Sherelee Holmes, who claims to be the great-great-granddaughter of the famous detective, has her first big case. And it's one that will take the keen mind of a Holmes to crack.
With the help of some real eccentrics (and her very own Watson), it is up to Sherelee to save her obnoxious sister Myra from TOP--the Temple of Perfection--whose Imperfect, Almost Perfect, Perfect, and Premium-Grade Brothers and Sisters are led by the most perfect one ever, Daddy Perfect.
But Daddy is a cunning foe. And rescuing Myra means infiltrating TOP--a place as crazy as a plate of scrambled eggs.
Jill Pinkwater has created a wonderfully unconventional novel that combines mystery, adventure and satire in one fast-paced story. . . with a solution that would have satisfied Sherlock himself.