동생 means "younger sibling regardless of sex" 아우 means "younger sibling of same sex" 남동생 means "younger brother" 여동생 means "younger sister" An elder sister calls her younger sister as 아우, 동생, 여동생 An elder brother calls his younger brother as 아우,동생, 남동생 An elder sister calls her younger brother as 동생, 남동생 An elder brother calls his younger sister as 동생, 여동생

Same meaning. but these days, we normally say '동생' not '아우'. 아우 sounds a little bit OLD.