If Amir knew him and Hassan were brothers, he would have still treated Hassan the same. It idn't matter if he knew they were brothers by blood, Amir was still afraid of Assef and that fact wouldn't change who he was frightened by. Amir says, "I ran because I was a coward. I was afraid of Assef and what he would do to me. I was afraid of getting hurt. That's what I told myself as I turned my back to the alley, to Hassan." The fact the he was his brother wouldn't change that he was still scared.