The Collaborative International Dictionary of English v.0.48:

Sib \Sib\ (s[i^]b), n. [AS. sibb alliance, gesib a relative.
   [root]289. See Gossip.]
   1. A blood relation. [Obs.] --Nash.
      [1913 Webster]

   2. a sibling.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Sib \Sib\, a.
   Related by blood; akin. [Obs. or Prov. Eng. & Scot.] --Sir W.
   Scott.
   [1913 Webster]

         Your kindred is but . . . little sib to you. --Chaucer.
   [1913 Webster]

         [He] is no fairy birn, ne sib at all
         To elfs, but sprung of seed terrestrial. --Spenser.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sib
    n 1: a person's brother or sister [syn: sibling, sib]
    2: one related by blood or origin; especially on sharing an
       ancestor with another [syn: blood relation, blood
       relative, cognate, sib]