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Title: The Deadly Sister
Author: Eliot Schrefer
Plot Summary from GoodReads:
Abby Goodwin is sure her sister Tabitha isn't a murderer. But her parents don't agree. Her friends don't agree. And the cops definitely don't agree. Tabitha is a drop-out, a stoner, a girl who's obsessed with her tutor, Clyde Andrews...until he ends up dead. Tabitha runs away, and leaves Abby following the trail of clues. Each piece of evidence points to Tabitha, but it also appears that Clyde had secrets of his own. And enemies. Like his brother, who Abby becomes involved with...until he falls under suspicion.
Is Abby getting closer to finding the true murderer? Or is someone leading her down a twisted false path?