성별에 따른 차이입니다. 누나는 남자의 손위 여성 언니는 여자의 손위 여성

Noona is what a male would call his older sister, and eunni is what a girl would call her older sister. This also works the same way for close friends.

Eonni = What girls say to older sisters or girls older than themselves. Noona = What boys say to older sisters or girls older than themselves