Big brother..? Am I missing something?

I think they may have known each other since childhood and he is older, hence "nii-san". I think it just sounds weird in English.

I'm guess it's how 'oppa' is used in korean. It's for big brother/older male. But women use it romanticaly too with their partner. But i guess men can use 'hyung' like that too. (Makes me think of Bj Alex)
Probably just language barrier