Source : Webster's Revised Unabridged Dictionary (1913)

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.

            I am the sister of one Claudio.       --Shak.

   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.

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

   {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}.

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

Source : WordNet®

sister
     n 1: a female person who has the same parents as another person;
          "my sister married a musician" [syn: {sis}] [ant: {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: sometimes used as a term of address for attractive young
        women [syn: {baby}]