Sarah is Lynn's beautiful elder half sister. She is 5 years older than Lynn. Cold and distant at their first meeting, Sarah is actually kind and caring. Gradually, she treats Lynn as if she is her own real younger sister, spurred on by their same state of being motherless, and defends her from the abuses of the Wibburys.
Unlike Lynn, Sarah doesn't have any memory of her mother since she died before she could even remember. Her mother's only keepsake is the flute and the musical score. She is very good at painting and an excellent flute-player. She has feelings for Arthur and is conscious of Lynn every time she saw them together

(Source: Wkipedia)