How do you say one little sister in Japanese?

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 妹 (imouto) are humble ways of saying little sister, used when little sister is your own or very familiar to you like a childhood friend, etc.

I haven't come across a uniquely "cute" way of saying little sister but you can add -san to make it respectful or -chan to make it friendly or somewhat cute.

Closest I can get you to it is with adding a little friendlier intonation to it, also adding 'yo' if you are calling to your little sister or close younger female friend; Like 'imouto-chan yo..!'