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Molly Moore is the younger sister of Poppy Moore and the youngest daughter of Mr. Moore and his first wife. Despite Poppy's (initially) "mean girl" attitude, Molly appears to be much nicer and not as spoiled/bratty as her older sister. Despite their (initially) vastly different personalities, Poppy and Molly appear to be quite close and get along pretty well, with Poppy even acting kind of as a motherly-figure to her sister (their actual mother had died in a car accident several before their start of the movie).
After her older sister plays a rather mean-spirited prank on their dad's girlfriend at the beginning of the movie, Gerry (their dad) punishes Poppy by sending her to an all-girls boarding school all the way over in England called Abbey Mount, and Molly seemed very sad about her sister leaving.
It's possible that Molly might end up going to Abbey Mount like her older sister and their mother (who's eventually revealed to have been a student there back in the 1970s).