형 means "a man's elder brother" 오빠 means "a woman's elder brother" 언니 means "a woman's elder sister" 누나 means "a man's elder sister" 동생 means "younger sibling regardless of sex" 아우 means "younger sibling of same sex" 남동생 means "younger brother" 여동생 means "younger sister" Thus, A woman calls her elder brother as 오빠 A man calls his elder brother as 형 A woman calls her elder sister as 언니 A man calls his elder sister as 누나 A woman calls her younger sister as 아우, 동생, 여동생 A man calls his younger brother as 아우,동생, 남동생 A woman calls her younger brother as 동생, 남동생 A man calls his younger sister as 동생, 여동생

To add to the first post, people do use these familial pronouns between non-family members as well. But you have to first establish age and sort of work out what you are going to call each other. Going up to someone that you don't know and calling them 언니/누나 may be seen as a bit rude and a bit like "I don't even know them and they are already calling me 누나. What does he want from me." What to call someone takes a long time to get used to for non-natives. One of the best ways to get used to this is to watch Korean dramas (when your Korean level is around mid to high intermediate). Then it will be easier to grasp how to address people in different situations.