What is revealed about Hassan’s parentage? Who can be inferred as his real parents?

The topics of secrets and family ties converge and come to a climax in Chapter Seventeen, when Rahim Khan finally reveals to Amir that Hassan was his half-brother. Amir is furious because suddenly the way he treated Hassan and Ali seems all the more wrong. The concept of "brother" is much stronger to him than the concept of "servant-best-friend" whereas Hassan had treated him like a brother no matter what. As Amir says, "Hassan had loved me once, loved me in a way that no one ever had or ever would again." Amir's reaction to the news reveals how important family ties are anywhere, but in Afghanistan particularly. Hassan and Amir shared the same father but with a different mother.