The word 妹 pertains to a person's "younger sister," as they use different words to address their family members. It has two characters, 女(おんな / onna), pertaining to the female gender and 未(み / mi), which can also mean "not yet" (also, the same kanji used in 未だ/まだ). "Woman not yet" is what perfectly describes a young female member of the family, thus, the word.