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]