one's own younger sister / the wife of one's younger brother; one's sister-in-law / a younger female / (archaic, said from a male speaker) one's sister regardless of age difference

younger sister

(literary) they (about women or girls)

one and only; unparalleled; matchless; without equal

younger paternal half sister

a wife's sister

(archaic, men's speech) an intimate form of address for a female by a male: "my sister"

someone else's younger sister

sworn younger sister