How should imouto address her brother?
I´ve got a rule: Every time I watch an anime with a character saying "onii-chan" I punch the keyboard and drop that anime immediately.
His most high and exalted imperial majesty, lord protector, defender of the faith, king of Rome, the illustrious and benevolent chosen god-emperor Oniisama the magnificent white emperor of the twelve divine heavens.
Every time. In full. No abbreviations.
Just going off of Kirino's popularity - 'loser', 'trash', and 'piece of shit' all seem to be popular choices.
Oh Shirou, never change.