The Collaborative International Dictionary of English v.0.48:

Sister \Sis"ter\, v. t.
   To be sister to; to resemble closely. [Obs.] --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sister \Sis"ter\, n. [OE. sister, fr. Icel. systir; also suster,
   from AS. sweostor, sweoster, swuster, akin to OFries.
   sweester, suster, LG. s["u]ster, suster, D. zuster, OS. &
   OHG. swestar, G. schwester, Icel. systir, Sw. syster, Dan.
   s["o]ster, Goth. swistar, Lith. ses?, Russ. sestra, Pol.
   siostra, L. soror, Skr. svasr. [root]298. Cf. Cousin.]
   1. A female who has the same parents with another person, or
      who has one of them only. In the latter case, she is more
      definitely called a half sister. The correlative of
      brother.
      [1913 Webster]

            I am the sister of one Claudio.       --Shak.
      [1913 Webster]

   2. A woman who is closely allied to, or assocciated with,
      another person, as in the sdame faith, society, order, or
      community. --James ii. 15.
      [1913 Webster]

   3. One of the same kind, or of the same condition; --
      generally used adjectively; as, sister fruits. --Pope.
      [1913 Webster]

   Sister Block (Naut.), a tackle block having two sheaves,
      one above the other.

   Sister hooks, a pair of hooks fitted together, the shank of
      one forming a mousing for the other; -- called also match
      hook.

   Sister of charity, Sister of mercy. (R. C. Ch.) See under
      Charity, and Mercy.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sister
    n 1: a female person who has the same parents as another person;
         "my sister married a musician" [syn: sister, sis] [ant:
         blood brother, brother]
    2: (Roman Catholic Church) a title given to a nun (and used as a
       form of address); "the Sisters taught her to love God"
    3: a female person who is a fellow member of a sorority or labor
       union or other group; "none of her sisters would betray her"
    4: (slang) sometimes used as a term of address for attractive
       young women [syn: baby, babe, sister]