Older guys and girls will call there younger sister 여동생 Yo dong seng
Older guys and girls will call there younger brother 남동생 nam dong seng
Younger one is 동생 dong seng
여자 is women or girl, which is shortened in this form
남자 is man but shortened to 남 in this situation
Girls will call older men (even if they are not actually siblings) 오빠 "oppa", meaning older brother
Girls will call older girls (again even if not related) as 언니 "eoni" meaning older sister
Guys will call older men (even if not related) 형 "hyeong" meaning older brother
Guys will call older girls (again even if not related) 누나 "Nuna"
these are all forms of respect since it is rude to call somebody older by the name
guys and girls will call women a lot older (im talking a LOT older) 아주마 a ju ma which means aunti
Guys and girls will call older men 아쩌시 a do shi.