누나 is used when a younger brother calls an elder sister and 언니 is used when a younger sister calls an elder sister.
However in korean culture 누나 is used when boys or men call girls or wowen who are older than them by showing intimacy
also 언니 is used when girls or women call girls or women who are older than them even they are not the blood relationship.
Thanks for the kind explanation. I could learn a lot about 누나 and 언니.