1. an elder sister.
2. (sometimes caps.) a woman who individually or as a member of an organized group undertakes to sponsor or assist a girl in need of help or guidance.
3. (sometimes caps.) a young woman in the junior or senior class in college who advises a young woman in the freshman class on studies, social activities, etc., often as a part of a sorority program.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.