Sibling
Old English, in the sense ‘relative’ (see sib, -ling). The current sense dates from the early 20th century.
wiktionary
1903, modern revival of Old English sibling(“relative, a relation, kinsman”), equivalent to sib + -ling. Compare Middle English sib, sibbe(“relative; kinsman”), German Sippe. The term apparently meant merely kin or relative until the 20th century when it was applied in a way that aided the study of genetics, which led to its specialized use. For example, the OED has a 1903 citation in which "sibling" must be defined for those who don't know the intended meaning. [1]
etymonline
sibling (n.)
"brother or sister," 1903, modern revival (in anthropology) of Old English sibling "relative, kinsman," from sibb "kinship, relationship; love, friendship, peace, happiness," from Proto-Germanic *sibja- "blood relation, relative," properly "one's own" (source also of Old Saxon sibba, Old Frisian, Middle Dutch sibbe, Old High German sippa, German Sippe, Gothic sibja "kin, kindred"), from PIE *s(w)e-bh(o)- (source also of Old Church Slavonic sobistvo, Russian sob "character, individuality"), an enlargement of the root *swe- "self" (see idiom). Related to the second element in gossip.
The word 'sib' or 'sibling' is coming into use in genetics in the English-speaking world, as an equivalent of the convenient German term 'Geschwister' [E.&C. Paul, "Human Heredity," 1930]
In Old English, sibb and its compounds covered grounds of "brotherly love, familial affection" which tended later to lump into love (n.), as in sibsumnes "peace, concord, brotherly love," sibbian (v.) "bring together, reconcile," sibbecoss "kiss of peace." Sibship, however, is a modern formation (1908). Sib persisted through Middle English as a noun, adjective, and verb expressing kinship and relationship.
