You're right. I guess what I was trying to say is that family titles (grandmother, aunt, uncle) etc are used far more often in Japan even when there is no family link between the two people. Brother might not be very commonly used in Japanese when there is no family link, but I don't think I've ever heard it used like that in English. In a lot of subs I've seen, even words like Oba-san are used when there is no family link, but the word isn't translated into aunt or even older woman (that sounds weird) they leave it at oba-san. I think this drama should leave the word onii-chan as onii-chan and not translate it, makes me a little uncomfortable to be honest.
Spoiler: ep 2
Spoiler: ep 3
Hehe, I wouldn't be surprised. I guess I meant not in shojou drama land.
Spoiler: ep 5