When Mom--a do-gooder who delegates the actual work of her philanthropic projects to others--announces that the family is taking in a needy child for the summer, daughter Ellis, 14, is dismayed; she's sure her own plans will be forfeited. And so they are, but not as she expects. Mom plans to park ten-year-old Lilac with Dad's parents, but then Gramps has a heart attack, and Ellis--with some help from her older sister and younger brother--becomes chief caretaker for this self-possessed waif from the potato fields of northern Maine. Everyone's so upset by Gramps' hospitalization that no one pays much attention to Lilac; her history comes out belatedly. Her novelist-to-be dad died four years ago; her overworked Mom is about to marry again and hopes Lilac wig be adopted. Believably, even these revelations don't immediately dispel the antagonism between Ellis (with her legitimate grievance) and Lilac (with her natural response to it); it takes a couple of extra twists to the lively plot to make them friends. Corcoran, author of 60+ books, holds attention. There's much that rings true here, especially the family too centered on its own middle-class preoccupations to be sensitive to the needs of a young outsider. The book would be stronger, however, if we knew how they got that way; Mom, especially, has no excuse but inherited snobbishness for her non-contributions to her own family as well as to Lilac. The outcome--Lilac will stay with the grandparents after all--is not wholly plausible. Still, warmhearted and entertaining.