'sibs' originates from Modern English informal short form 'sib' (short for 'sibling'), which in turn comes from Old English 'sibb', where 'sibb' meant 'kinship, relationship'.

'sibs' developed as the colloquial plural of the short form 'sib' (from 'sibling'). The element 'sib' comes from Old English 'sibb' (Middle English forms included 'sibbe'), and the derivative 'sibling' formed with the suffix '-ling' led to the modern family of words; the colloquial short form 'sib' then produced the plural 'sibs'.

Initially 'sibb' meant 'kinship' or 'relationship' in Old English; over time the sense narrowed to refer specifically to a person's brothers and sisters, and in modern informal usage 'sibs' denotes those siblings.