언니 is a word used a lot among young girls. Basically, it is used between siblings. 언니 is the word a younger brother uses for an older sister.
It is also a word used for women who are older than themselves at school, work, or clubs.
However, it can sometimes be used in informal situations for women younger than themselves. Since 언니 is an informal word, it is not recommended to use it in situations where dignity must be respected.