"He and she are siblings."
Translation:Lui e lei sono fratelli.
sibling goes back to Old English but was no longer used until reintroduced back at the beginning of the 20th century as an anthropological term. In German, one says Geschwister, cf. Schwester 'sister', though the term is sex-neutral. In Japanese, one says 兄弟 'kyoudai', a Sino-Japanese term literally meaning big-brother/little brother. But if one is talking about two or more sisters, the term is 姉妹 shimai, lit. 'older sister/younger sister'.