Definitions of sister:

• noun:   a female person who is a fellow member of a sorority or labor union or other group

  Example: "None of her sisters would betray her"

• noun:   (Roman Catholic Church) a title given to a nun (and used as a form of address)

  Example: "The Sisters taught her to love God"

• noun:   a female person who has the same parents as another person

  Example: "My sister married a musician"

• noun:   sometimes used as a term of address for attractive young women