big sister

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An older sister.
• noun A woman who assumes the role of an older sister, as by providing guidance or protection.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A sibling's older sister (used especially by children or by parents in speaking to their children).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an older sister