妹 mèi

: younger sister

Kun Reading

妹 | いもうと imouto

: (Humble (kenjougo) language) younger sister

妹さん | いもうとさん imouto san

: (Honorific or respectful language) younger sister

妹 | いも imo

: (archaism) (familiar language) darling (only of a woman); dear; honey; my betrothed
: (archaism) one’s sister;
: (archaism) (familiar language) (feminine speech) friend

On Reading

まい mai


妹妹 mei4mei: younger sister

妹夫 mei4fu: younger sister’ husband
姐妹 jie3mei4: sisters
堂妹 tang2 mei4: younger female patrilineal cousin
表妹 biao3mei4: younger female cousin matriarchal line
姊妹 zi3mei4: sisters
兄妹 xiong1mei4: brothers & sisters
[けいまい keimai OR きょうだい kyoudai]
辣妹 la4mei4: hot chick; sexy girl
台妹 tai2mei4: local girl (Taiwanese)

恐龍妹 [恐龙妹] kong3 long2 mei4: ugly girl (slang)

形声 Pictophonetic.
女 (nü3: woman/feminine rad 38) suggests the meaning while
未 (wei4: not yet) provides the sound.

wo3 mei4mei shi4 wo3de fu4mu3 de bao3bei4.
My younger sister is my parents’ (precious) treasure.

Resources: Chinese Dictionary | Japanese Dictionary