A polite way to refer to someone's younger sister; used as an honorific form when speaking about another person's sister.
田中さんの妹さんは医者だと聞きました。
I heard that Mr. Tanaka's younger sister is a doctor.
妹さんは今どこで働いていますか？
Where does your sister work now?
あの方の妹さんはとても礼儀正しいです。
That person's younger sister is very polite.