sib·ship \ˈsibˌship\noun Etymology:sib (III) + -ship 1.: the quality or state of being a sib or a member of a sib 2. a.: sib 3; broadly: kindred b.: a group of sibs : kinship < of 16 sibship groups, 13 consisted of two and 3 of three siblings each — J.H.Conn & Leo Kanner >