The Lost Twin by Sophie Cleverly

Ivy has been summoned to Brookstone. Her twin sister Scarlet, the more vibrant twin, has died of the flu and Ivy is to replace her. Literally. She is to <i>become</i> Scarlet. But that seems like an impossible mission even before Ivy finds Scarlet's diary. Is it possible that there is more to the story? And what is the mysterious reason that Penny seems to hate Scarlet so very much?
By becoming Scarlet, Ivy finds reserves within herself that she never knew she had. Up to and including standing up to the very mean Penny and the truly terrifying headmistress.
A fast and fun read. A mostly satisfying ending but I was glad to see that this was only the first in a series since there are some threads left undone.