An example of a sibling is the little boy born to your mother and father right after you. He is your brother and your sibling.
Origin of sibling20th-c. revival of OE, a relative: see sib and amp; -ling
Origin of siblingMiddle English, from Old English, from sibb, kinsman; see sib.
1903, modern revival of Old English sibling (“relative, a relation, kinsman"), equivalent to sib +"Ž -ling. Compare Middle English sib, sibbe (“relative, kinsman"). The term apparently meant merely kin or relative until the 20th century when its necessity for the study of genetics 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.
- "1903 K. PEARSON in Biometrika II. 369 These [calculations] will enable us..to predict the probable character in any individual from a knowledge of one or more parents or brethren ("˜siblings', = brothers or sisters)." ("Sibling," OED.)