妹 is a bound form (b.f.) meaning 'younger sister'. It cannot be used alone as a word; instead, it appears as a morpheme in compounds such as 姐妹 (sisters), 表妹 (younger female cousin), or 师妹 (junior female apprentice). When you see 妹 in a word, it always carries the 'younger sister' sense, but you must attach it before or after another morpheme to form a complete noun.