1One's younger sister.
私には妹が一人います。
I have one younger sister.
妹は今年から中学生になった。
My younger sister started junior high this year.
妹とけんかしたけど、すぐに仲直りした。
I had a fight with my younger sister, but we made up quickly.
田中さんの妹はピアノがとても上手だ。
Mr. Tanaka's younger sister is very good at playing the piano.