1903, modern revival of Old Englishsibling(“relative, a relation, kinsman”), equivalent to sib +-ling. Compare Middle Englishsib, 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.)