How that is weird? There are some words to call someone order/younger then you instead of calling their names. So if you are a female and you have a order bother, you call him the same way. Simply because he is older then you. However when you introduce your older bother, you can use 친오빠. It means one's own brother. 누나/오빠 is just the most basic way to call someone order then you. Some girls call their boyfriend 오빠 because their boyfriends are order then them. Is not like darling, sweetie, hon, etc. What do you thinking?