訓 いもうと ・ 音 マイ
漢越音: muội
The 女 (woman) radical with 未 (not yet): the girl who is not yet grown, which is exactly the younger sister.
Stroke data: KanjiVG (CC BY-SA 3.0) — kanjivg.tagaini.net
妹はまだ子供です。 (いもうとはまだこどもです)
My younger sister is still a child.