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]

Moby Thesaurus II by Grady Ward, 1.0:

161 Moby Thesaurus words for "sister":
   Greek, LPN, RN, abbess, affiliate, ally, alter ego, analogon,
   analogue, associate, aunt, auntie, belonger, blood brother,
   brethren, brother, bub, bubba, bud, buddy, canoness, card-carrier,
   card-carrying member, cardholder, catechumen, charge nurse,
   charter member, church member, churchman, churchwoman, clergywoman,
   close copy, close match, clubber, clubman, clubwoman, cognate,
   committeeman, communicant, companion, complement, comrade,
   congenator, congener, conventioneer, conventioner, conventionist,
   conventual, coordinate, correlate, correlative, correspondent,
   counterpart, country cousin, cousin, cousin once removed,
   cousin twice removed, daughter, district nurse, dues-paying member,
   enlistee, enrollee, equivalent, father, fellow, first cousin,
   foster brother, frater, fraternity man, graduate nurse,
   grandnephew, grandniece, granduncle, great-aunt, great-uncle,
   guildsman, half brother, honorary member, image, initiate, insider,
   joiner, kid brother, kindred spirit, lady superior, laic,
   lay brother, lay sister, layman, laywoman,
   licensed practical nurse, life member, like, likeness, mate,
   member, mother, mother superior, near duplicate, nephew, niece,
   novice, nun, nuncle, nunks, nunky, nurse, nursing sister, obverse,
   one of us, parallel, parishioner, pendant, picture, pledge,
   postulant, practical nurse, prioress, private-duty nurse,
   probationer, probationist, probe, public health nurse, reciprocal,
   registered nurse, religieuse, school nurse, scrub nurse,
   second cousin, second self, secular, secular canoness, similitude,
   simulacrum, sis, sissy, sister-german, sistern, socius, son,
   sorority girl, sorority woman, soul mate, stepbrother, stepsister,
   student nurse, such, suchlike, superioress, surgical nurse, tally,
   the like of, the likes of, the reverend mother, trained nurse,
   twin, unc, uncle, uncs, uterine brother, visiting nurse



Bouvier's Law Dictionary, Revised 6th Ed (1856):

SISTER. A woman who has the same father and mother with another, or has one
of them only. In the first case she is called sister, simply; in the second,
half sister. Vide Brother; Children; Descent; Father; Mother.