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 seem to get along pretty well and appear to be very close, with Poppy even acting as somewhat of a motherly-figure to her sister (due to the death of their mom, who had been killed in a car accident several years before the start of the movie).
After Poppy plays a mean-spirited prank on their dad's girlfriend at the beginning of the movie, Gerry (Poppy and Molly's dad) punishes Poppy by having her sent to an all-girls boarding school all the way over in England called Abbey Mount. 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 was a student at the school back in the 1970s).