sibling [sub + -ling.] (s&ibreve_;bl&ibreve_;ng), n. a brother or a sister. 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]