Who is Third Sister from Snow Flower and the Secret Fan and what is their importance?

Third Sister is Lily's sister, who is a year younger than Lily, is scheduled to have her feet bound at the same time as Lily and Beautiful Moon. However, this sister cannot face the torture she is about to endure, and she runs from her mother, terrified, and fights the process the entire time. When her foot bones are supposed to be breaking, she instead develops blood poisoning and dies. Her death makes Lily's mother more conscientious about the process, and makes Lily realize how seriously bad it can go if not done correctly.