Josey Muller's regular life—studying with her best friends, planning for the Hollidazzle dance, and gossiping about hot guys—is shaken like a snow globe when her parents drop an A-bomb: she has a sister! Pressured to give up the baby they conceived in high school, her parents are overjoyed to reunite with Audrey. Even Josey's brothers are cool with it.

No longer the only daughter and the "smart one" in the family, Josey struggles to accept her infuriatingly friendly, witty, and talented older sister. But feelings of betrayal and jealousy threaten to boil over when she learns that Audrey is on her way to becoming a psychologist, Josey's life goal. Just when she's given up on ever feeling like a sister to this stranger, a new side of Audrey is revealed ... and their real-life nature versus nurture experiment offers a fresh start for them both.