sister

Concise Oxford English Dictionary © 2008 Oxford University Press:
sister/ˈsɪstə(r)/

▶noun

• 1 a woman or girl in relation to other daughters and sons of her parents.

• 2 a female friend or associate.

  ■ a fellow woman.

  ■ N. Amer. a fellow black woman.

• 3 a member of a religious order of women.

• 4 (often Sister) Brit. a senior female nurse.

• 5 [as modifier] denoting an organization or a place which bears a relationship to another of common origin or allegiance.

– derivatives
sisterliness noun,
sisterly adjective.

– origin OE, of Gmc origin.