The correct translation should be "you consider me as (or like) your brother" that is the proper and grammatically correct sentence in proper English. This seems to happen a lot on here now, the correct translation is wrong because it's not in proper (correctly structured sentence in) English and is grammatically incorrect, as they often leave out words like "that" and "as" etc, which are essential for the proper structuring of a sentence in English
Yes, the "as" could be correct -- "You consider me (as if I were) your brother." But notice the "Usted" subject and the "su" (not "tu"). So the situation could be a mistaken identity - "You (not familiar) consider me to be his brother (but I'm not)." Or maybe it's just another Duo sentence and who knows! I love this program.