The character 妹 (mèi) itself means "younger sister". The word 妹妹 is formed by simply repeating this single character, a common method in Chinese for creating familiar or affectionate terms for family members and close relationships. This reduplication creates a softer, more intimate sound and feel than the single character alone, transforming the basic idea of "younger sister" into the everyday, affectionate term used to address or refer to her.