Responses
I think he rather used "hyung" wich clearly means big brother
no... he doesn't think of them in a romantic way, especially because he was addressing BOTH of them. he meant it as big brothers or siblings like he mentioned earlier. probably hyung.
I think that he is addressing only her... not both of them...
He was touching her forehead and that's why I thought it... again, my impressions only.
I'm pretty sure that when he said "let big brother solve this" he were using "oppa" in korean which can be translated as "big brother" or as 《romantic partner》.