A Summer to Die — Lois Lowry

When Meg's family moves to a small house in the country, she must get used to many new things. She's annoyed about being stuck sharing a bedroom with her older sister, Molly. They have no common interests, and it's hard for Meg to hide her resentment of Molly's beauty and popularity.

Just as Meg begins adjusting to her new surroundings, she feels that Molly is being a real nuisance again. But it's soon evident that Molly's constant grouchiness, changing appearance, and other complaints are not just part of a new mood. The day Molly is rushed to the hospital, Meg has to accept the fact that something is terribly wrong with her sister. On that day, Meg's world changes forever. And although she's hurting and frightened, maybe it's not too late for Meg to show Molly what she truly feels.