sib 
(sĭb)
(sĭb)n.
1.
a. A blood relation; a relative.
b. A person's relatives considered as a group; kinfolk.
2. A brother or sister; a sibling.
3. Anthropology A kinship group consisting of two or more lineages considered as being related, as by common descent from a mythic ancestor.
adj.
Related by blood; kindred.
[Middle English sibbe, from Old English sibb; see s(w)e- in the Appendix of Indo-European roots.]