The Korean system of referring to different family members is complicated:
Are you male or female? Are they male or female? Are they older or younger than you? Formal or informal?
So here is a rough guide on immediate family~
동생: younger sibling
남동생: younger brother
여동생: younger sister
누나: older sister (if male speaker)
언니: older sister (if female speaker)
형: older brother (if male speaker)
오빠: older brother (if female speaker)
Note// The usage of the words for older and younger siblings is not limited to blood relatives, it can be used for friends, waiters/waitresses etc (but not if the other person is significantly older than you).