for the sister (nuna) and for the brother (oppa or elder brother hyung)

There are different words to use if you (the speaker) are a man or woman. Also, Koreans usually call their friends "brother" or "sister", so you should add 친 (chin) before the words below to indicate that it is their REAL family, or just leave as it is for a male or female friend: e.g. 친누나 (chin-nu-na) = My real older sister. For MEN speaking about their family/friends: 1. elder brother = 형 (hyung) 2. younger brother = 남동생* (nam-dong-seng) 3. elder sister = 누나 (nuna) 4. younger sister = 여동생* (yeo-dong-seng) For WOMEN speaking about their family/friends: 1. elder brother = 오빠 (o-bba) 2. younger brother = 남동생* (nam-dong-seng) 3. older sister = 언니 (eon-ni) 4. younger sister = 여동생* (yeo-dong-seng) P.S. 남동생/여동생(nam-dong-seng/yeo-dong-seng) are usually just called 동생 (dong-seng) when referring to someone younger than you - their gender 남/여 (male=nam/female=yeo) is usually omitted.