It is a spin-off to The Sisterhood of the Traveling Pants series as main characters Polly, Jo and Ama are about to attend South Bethesda High School, the school the original sisterhood (Bridget "Bee" Vreeland, Lena Kaligaris, Carmen Lowell and Tibby Rollins) attended.
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Effie, Lena's younger sister plays a significant role in the book.
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Tibby is mentioned by Polly, because she babysits for Nicky and Katherine, Tibby's younger siblings.
In 3 Willows, Polly babysits Tibby's little brother and sister.
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