Example sentences
mèi妹mei妹xǐ喜huan欢chàng唱gē歌。
My little sister likes singing.
mèi妹mei妹shuì睡zháo着le了。
My little sister fell asleep.
About this word
妹 is "younger sister", and the whole word is that syllable twice with the second one flattened to a neutral tone. Every sibling word in Chinese works this way: 哥哥, 姐姐, 弟弟. Notice what English hides and Chinese forces: 姐妹 means sisters as a pair, but 妹妹 is one younger sister, and there is no neutral word for "sister" to fall back on. Outside the family, 小妹妹 is how an adult addresses a little girl, and 妹妹 can also be a soft way to speak of a younger woman you are close to.
The story behind the character
妹
妹 is 女 (woman) with 未 on the right giving the sound. Xu Shen's definition runs three characters: 妹，女弟也, a female younger sibling. The character has never meant anything else. Sometimes a whole history fits in one line.