you know, bad or good to her, if she calls her loveer "brother", that's already fucked up
It's also a generic term for males old enough to be one's older brother.
I mean, I doubt that's what's intended here, but, y'know, food for thought.
Ahaha, onii-chan!Talking to her loverBeing praisedEyes accidentally meetBeing told "I love you"Ahaha, looks like our eyes met, onii-chan...Ehehe... thank you, onii-chan.Being pattedLet's play, let's play!Ah...Eh!?Ehe, ehehehehe...Yeah, I love you too, onii-chan!So this is what love is... and I thought you only thought of me as a little sister...Practice expressions of your wife being lovedYeah, I love you too... because you're my onii-chan...Being told "let's go out"Mm, petting?Fufu... do it more, onii-chan.