How does Amir’s new understanding of Hassan’s parentage change his image of Baba? How does this revelation impact Amir?

Now that he knew Hassan was his half-brother, it seemed absurd that he had not realized it before. Baba had always treated Hassan like a son not just because he cared for him, but because Hassan was really his son. Amir wondered how Baba could have broken his own cardinal rule about not lying, how he could have lived with himself after shaming Ali.

Suddenly, Baba did not seem like such a shining example of righteousness. Amir now understood that Rahim Khan had called him to Peshawar to pay not only for his betrayal of Hassan, but for Baba's betrayal of Ali. Amir wondered if he was to blame for Hassan and Ali's deaths because he was the one who drove them out of the house and split up the family.