A Summer to die / by Lois Lowry.

'Funny thing about sisters. Sometimes it seems as if, when our parents created us, it took them two tries, two daughters, to get all the qualities of one whole, well-put-together person.' Meg and Molly couldn't be more different. Pretty and popular, Molly is the one who always knows what to say, who is fun and full of smiles. She's sure about the future, has sorted out her goals, while Meg, determined and unsure at the same time, is sometimes angry over nothing, often miserable about everything. Then Molly gets sick and the world flips upside down. Now Molly gets sick and the world flips upside down. Now Molly is the grouchy one, throwing tantrums at the slightest provocation, while their parents sit by silently. Meg is confused by the changes in Molly and their parents, until she realises why.