What type of differences do think would have taken place if Amir was aware from the very beginning that Hassan was his brother?

THis is difficult to say. I doubt they would have been as close as they were. Knowing that Hassan was his half brother would have changed their dynamica as well as Amir's dynamics with his father.

Amir would probably have protected Hassan and probably become close to him. He might've protected Hassan when Assef and his gang assaulted him. He wouldn't have held the guild. That would ruin the whole story I think.