sibling

Etymology

1903, modern revival of Old English sibling (“relative, a relation, kinsman”), equivalent to sib + -ling. Compare Middle English siblynges pl, 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.

noun

1. A person who shares a parent; one's brother or sister who one shares a parent with.
   None of my siblings are married yet.
2. (computing theory) A node in a data structure that shares its parent with another node.
3. (taxonomy) The most closely related species, or one of several most closely related species when none can be determined to be more closely related.
   D. simulans, D. mauritiana and D. sechellia (which we will call “the siblings” when we do not need to distinguish among them) have homosequential polytene chromosome banding patterns differing from those of D. melanogaster by one long inversion on chromosome arm 3R and a few much smaller inversions. April 1, 1990, P. Hutter et al., “A genetic basis for the inviability of hybrids between sibling species of Drosophila”, in Genetics, volume 124, number 4, pages 909–920
   Bush suggested that this difference might represent a case of chromosomal polymorphism or, more likely, sibling species. November 26, 2015, Lucie Vaníčková et al., “Current knowledge of the species complex Anastrepha fraterculus (Diptera, Tephritidae) in Brazil”, in ZooKeys, volume 540, →DOI, pages 211–237