sibling

n. brother or sister, one who has the same parents as another person

One's sibling is one's brother or sister, respectively meaning a male or female with whom one shares at least one parent. This is usually taken to mean that the two people are genetically very close, though it is not always necessarily the case, for example one or more siblings may have been adopted by their parents.
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Sibling /sib·ling/ any of two or more offspring of the same parents; a brother or sister. sib·ling n. [more]Sibling - Community and Resources

Noun
1. a person's brother or sister
(synonym) sib
(hypernym) relative, relation
(hyponym) half blood
(member-holonym) family, family unit

Synonyms and related words:
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