Baba deprives Amir of a brother. He hides the fact that he is in actuality Hassan's father. Amir is constantly seeking the love and affection of his father, whom he fears blames him for the death of his mother. Perhaps if Baba had recognized Hassan as his father, Amir would have felt he had more of a "family". Rahim Khan, Baba's close friend, understands and sympathizes with Amir. He is the main reason Amir returns to Afghanistan years later. Amir learns that Hassan was not only his best friend, but also his brother. He also tracks down Hassan's son, whom he adopts and loves as his own. While Baba deprived Amir of a brother, Rahim gave Amir his nephew and adopted son.
In the book "The Kite Runner" Amir grew up hanging out with his friend/servant Hassan. Hassan is a young Hazara boy. Baba, Amir's father, impregnated Hassan's mother. She was married to his childhood friend and now his servant, Ali.
Baba was ashamed by his actions and could not come out publicly about them. He is a well respected member in his community and the idea of him lying with a Hazara woman would be shameful. Baba keeps Ali on as a servant and continues to provide for both sons. His legitimate son, Amir, gets the benefit of Baba's wealth. Yet, his son, Hassan, is the one whose courage and personality he is most proud of.
Baba dies without ever telling Amir that he has a brother.
Rahim sends a letter telling Amir that he is dying. Amir flies to Afghanistan to see him. Rahim tells him the story about Hassan. Amir learns that Hassan and his wife were murdered by the Taliban. Rahim gives him money and helps to arrange for Amir to rescue Hassan's son. Amir originally was going to place the boy in an orphanage after he rescues him. However, he begins to realize how much they need each other. He gains a nephew who he will raise in his home in America.