The Collaborative International Dictionary of English v.0.48:

sibling \sib"ling\ (s[i^]b"l[i^]ng), a.
   of or pertaining to a sibling, n.; as, sibling rivalry: the
   common rivalry between siblings.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

sibling \sib"ling\ [sub + -ling.] (s[i^]b"l[i^]ng), n.
   a brother or a sister.

   Note: Siblings have at least one parent in common. Those
         related only by a common mother are uterine siblings;
         those related only by a common father are agnate
         siblings or consanguine siblings (a legal term). A
         sibling having both parents in common is a
         sibling-german or a full brother or full sister.
         These modifying terms are more commonly used for the
         more specific uterine brother, uterine sister,
         agnate brother, brother-german, etc.
         [PJC]

WordNet (r) 3.0 (2006):

sibling
    n 1: a person's brother or sister [syn: sibling, sib]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "sibling":
   affiliated, agnate, akin, allied, ancestry, avuncular, blood,
   blood relation, blood relative, clansman, closely related, cognate,
   collateral, collateral relative, congeneric, connections,
   consanguine, consanguinean, consanguineous, distaff side,
   distant relation, distantly related, enate, family, flesh,
   flesh and blood, folks, foster, german, germane, kin, kindred,
   kinfolk, kinnery, kinsfolk, kinsman, kinsmen, kinswoman,
   kith and kin, matrilateral, matrilineal, matroclinous,
   near relation, next of kin, novercal, of the blood, patrilateral,
   patrilineal, patroclinous, people, posterity, related, relations,
   relatives, sib, spear kin, spear side, spindle kin, spindle side,
   sword side, tribesman, uterine, uterine kin