Responses
East Asian cultures refer to close relationships as family members. Thing, Oops, Onii-chan, etc can mean big brother (blood related) or it can also mean someone that is an older brother's age range from them. So the two characters are not blood related or step siblings or whatever else, just close friends.
Hyung, Oppa, Onii-chan*
bro im so confused if it’s incest or not it seems like it to me but tbh i am so confused by the whole thing ╥﹏╥