Responses
I agree, it's basically like a nickname
I think it's a matter of translation..and possibly culture as well. In my country too we have the same nicknames that can be used both between siblings and couples..
In Korea, its normal to call someone thats older and close to you "hyung" which is older brother, even though they are not your brother. The reverse of that would be "dongsaeng" which is for your younger sibling or someone who is close to you and younger. So pretty much its not some incest shit but more of like a way to signify that they are close