I can already see where this is going.
They start with the Onii-chan thing.
They start being embarassed and find themselves in “dangerous” situations.
They start to love each other, without telling the other one for a while.
They notice and are ready to become a couple.
Real brother comes back and she’s indecisive.
S*it happens.
I’m not sure if there will be a “real brother is not related by blood” thing too, things will get messy in this scenario because Maria could really date him or have a relationship with Onii in that case.