妹 is one of the most commonly used Japanese kanji, and it also has a lot of special readings in addition to the standard onyomi and kunyomi readings.
ONYOMI 音読み
いもうと imouto
KUNYOMI 訓読み
マイ mai
Examples
あのかわいい少女は私の妹です。That pretty girl is my sister.妹は足が長い。My sister has long legs.私の妹は有名だ。My sister is famous.彼は妹がいません。He doesn’t have a sister.私の妹はよく泣く。My sister often cries.私には妹がいません。I do not have a sister.妹は人形で遊んでいる。My sister is playing with a doll.妹は21で大学生です。My sister is twenty-one years old and a college student.彼女の妹はとても好き。I like her sister very much.彼は妹を連れて行った。He took his sister along.