The word "姐妹" is formed by combining two individual characters, each representing a specific familial role: "姐" (jiě), meaning "older sister", and "妹" (mèi), meaning "younger sister". By joining them, the compound creates a single, cohesive concept that encompasses the general idea of "sisters" as a collective or a pair, moving from the specific distinctions of age to a unified term denoting siblinghood and close female kinship.