noun
little sister; younger sister
Neutral noun for a younger sister. Not inherently humble; used in general descriptions.
彼女は小妹と一緒に住んでいる。
She lives with her younger sister.
noun
my younger sister
Humble term used when referring to one's own younger sister to someone outside the family.
小妹はまだ学生でございます。
My younger sister is still a student.
pronoun
Humble first-person pronoun used mainly by young women in letters or formal writing. Rare in modern spoken Japanese.
小妹は先日、京都へ参りました。
I went to Kyoto the other day.
Compound of 小 (small, young) and 妹 (younger sister). The pronoun usage likely developed from the humble reference to one's own younger sister.