妹
いもうと · imouto · JLPT N5 · noun
sister
👧 妹 = 女 woman + 未 not yet: the girl who is not yet grown
The left part 女 means woman; the right part 未 means not yet and also hints at the sound. A woman who is not yet grown up is the younger sister. Use imouto for your own sister; someone else's is imouto-san.