Well, we use onii-chan for males who are slightly older than us, it's not just for familial relationship. Just like using oji-san (uncle) to old male strangers.
I mean it would be more strange (from her perspective) if she stopped calling him oni-san, even in manhwas they all call older male 'hyung/oppa' all the time whether they are family or not
I get what your saying hahahahah
But I don't deny that I feel slightly guilty liking this story. Not that it stopped me from reading the novel and the manga, but I would feel better if the oni-chan thing went away (￣∇￣")
She could just stop calling him oni-chan... It would be great for my conscience