Is it farther or further?
"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" (cf. 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)- (cf. 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.), e.g. 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.
sibling sib·ling (sĭb'lĭng)
One of two or more individuals having one or both parents in common; a brother or sister.
typically, a brother or a sister. Many societies choose not to differentiate children who have both parents in common from those who share only one parent; all are known simply as siblings. In those societies that do differentiate children on this basis, the former are known as full siblings, and the latter are known as half-siblings. Siblings may be the biological offspring of their parents, or they may be socially classified as such through adoption or the categories used in various descent systems. For instance, in some societies the relationships between certain sets of cousins (most often parallel cousins, the children of one's mother's sister or father's brother) may be the same as those that other forms of reckoning expect between biological siblings. In European and related traditions, the study of child development has included sibling relationships as important factors in personality formation. In many traditional cultures, the rights and obligations that obtain between full siblings are among the most sacrosanct of all the ties that bind kinship groups together.