noona and unni are basically the same, meaning older sister, but it differs from the speaker. if a boy calls the older sister "noona", the girl calls the older sister "unni". but nowadays people can switch it like the boy can call her unni, and a girl can call a boy older than her as "hyung". it represents their closeness.

Hi, a girl is called noona by a boy younger than her, and called unni by a girl younger than her, they can either be siblings or friends :)