Imooto (いもうと, pronounced "ee-moe-toe", with emphasis on the "mo") means little sister. Add -san to the end of imooto when it is someone else's little sister.
The Japanese word for 'little sister' is "Imōto" (妹) = little sister If you intended to say "one little sister" it would be said "Imōto ga hitori" or "Imōto ga ichi nin."
妹 is the kanji for little sister with different readings and usages. Most frequent readings of it are 'imouto' (kun-yomi=Japanese reading) and 'mai' (on-yomi=Chinese reading). 愚妹 (gumai) and...