Who is Aunt Hepsa from Calico Bush and what is their importance?

Aunt Hepsa lives on Sunday Island with her nephew Seth and grand-nephew Ethan. She was once married and had two sons of her own, but they all three died before the opening pages of this novel. Aunt Hepsa has lived with Seth for many years, helping to raise Ethan after his own mother's death. Now Aunt Hepsa lives happily on Sunday Island dyeing and weaving wool, as well as caring for the sick. Aunt Hepsa quickly becomes Marguerite's friend and surrogate mother, helping her every chance she gets.