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