"My older sister is learning Korean."
Translation:언니는 한국어를 배우고 있어요.
The gender of the speaker is not even mentioned. How can we know if 언니 or 누나 should we use? 누나 should be accepted too. Unless, it is specified that the speaker is female.
Yes, 누나 can be a possible answer, however, it should have 가 to indicate as the subject/topic