The Summer of Mrs. MacGregor by Betty Ren Wright

A girl learns that glamour isn't always what it seems: "Well written with humor, strong characterization, and a convincing plot" (School Library Journal).
Twelve-year-old Caroline feels torn. She grieves over her sister who has a severe heart problem that keeps her in and out of the hospital. But at the same time, she is jealous of her sister's beauty and sweet disposition, which causes everyone to admire her. Caroline longs to be beautiful, to get attention from her parents-just to be somebody, anybody, other than plain old Caroline Cabot.

Her self-image changes, however, when she meets glamorous seventeen-year-old Lillina MacGregor. Lillina makes her feel like somebody, but does Lillina tell the truth? Is she really 17? And married? And a model in New York? Caroline must rely on her inner strengths as she discovers the truth about Lillina, and will face several challenging situations during "the summer of Mrs. MacGregor."