A younger sister — a female sibling who is younger than oneself.
私の妹は高校生で、毎朝駅まで自転車で通っている。
My younger sister is a high school student and cycles to the station every morning.
妹にプレゼントを買ってあげたら、とても喜んでくれた。
I bought a present for my younger sister and she was very pleased.
兄は妹の宿題を手伝うことがよくある。
The older brother often helps his younger sister with homework.
うちの妹は料理が得意で、よく夕食を作ってくれる。
My younger sister is good at cooking and often makes dinner.