a blood relative; kinsman or kinswoman; esp., a brother or sister
Webster’s New World College Dictionary, 4th Edition. Copyright © 2010 by
Houghton Mifflin Harcourt. All rights reserved.
Word origin of 'sib'ME sibb < OE, kinsman, kinship, akin to Ger sippe, kinship < IE *s(w)ebh-, of the same kind (> OHG Swaba, Swabian) < base *se-, apart > L se