Who is Anne's sister from Gift from the Sea and what is their importance?

Anne's Sister is another unnamed member of the family, Anne's sister represents the ideal of being able to live with another while maintaining one's self. Together, she and Anne co-exist for a week, spending time together in conversation and activity, yet allowing the other to exist in her own fashion. Each does her own thing with work and rest, and the space and the lack of clinging contributes to a fulfilling and joyous relationship.