It is the person who is the daughter of the same parents with respect to another. Also if they share only the father or mother, although in these cases there is the locution "half-sibling". By extension it is used as an adjective for someone very dear, like a true blood brother. And the origin is really in an adjective, since in Latin the expressions frater germanus ("carnal brother") and soror germana ("carnal sister") were used, which lost the noun and evolved from "carnal, germinal, of origin" which only marked the character of blood family. See fraternity, sorority, sorority, sorority.
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