Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
sibling (n.)
Originally a rare Middle English word meaning “kin, a relative,” sibling became obsolete until the twentieth century, when the biological and social sciences brought it back, first as a generic term meaning “blood relative” and then as a specialized term meaning either “brother or sister,” as in She has three siblings—two brothers and a sister. For some time it seemed to be limited to scholarly and technical use, but today its inclusiveness (see INCLUSIVE LANGUAGE) has made it handy in the law, in journalism, and in conversation. Sibling is related to sib, but the latter can mean any blood kindred group, not just brothers and sisters; sib is not a clipped form of sibling, although it is sometimes erroneously used as though it were. Sibling is Standard, but be wary of annoying those few conservatives who still consider it jargon.
