Pardon? I don't quite understand. What phrase is old-fashioned? I think there's some info missing in your question ^^

As Sai said - your question is not very clear. It helps to put phrases in quote marks. If you meant 'Is the phrase "I thought you your brother" old-fashioned and what does it mean?' - then yes, that construction is old-fashioned. It is short for 'I thought you were your brother' - in other words, I mistook you for your brother.