What else does amir learn about Hassan/baba/ali?

After Rahim Khan finished telling the story about him and Hassan, he handed Amir a letter and a photograph. The photograph was of Hassan and Sohrab. In the letter, Hassan described the violence and injustice in Afghanistan. One day, Farzana spoke slightly loudly in the market and a Talib beat her so hard that she fell down and was bruised for days. Despite the terror, Hassan said, Sohrab was a healthy and smart boy; Hassan had made sure he was literate and knew how to shoot a slingshot as well as his father. Hassan ended his letter by expressing his wish to see Amir in Afghanistan again. Rahim Khan explained that the letter was written six months before. A month after he had arrived in Peshawar, he received news of Hassan's death from a friend. After he left Kabul, word spread that a Hazara family was living alone in Baba's house. One day, the Taliban came to the house and demanded that they leave. When Hassan protested, they took him out to the street, forced him to kneel, and shot him in the back of his head. Farzana ran out screaming and they shot her dead as well. The news devastated Amir, who could only whisper, "No. No. No."

Rahim Khan explained that the Taliban now occupied Baba's house and they were not held accountable for Hassan and Farzana's murders. Then he told Amir that he real reason he made Amir come to Peshawar was to bring Sohrab there. An American couple named Thomas and Betty Caldwell ran a goodwill organization there and would take care of him. When Amir protested and suggested Rahim Khan hire someone to find Sohrab, Rahim Khan was insulted. He told Amir, "I think we both know why it has to be you, don't we?" Then he asked Amir if he had become what Baba feared so many years before, a person who "can't stand up to anything." He said it was his dying wish for Amir personally to bring Sohrab to Peshawar. When Amir continued to refuse, Rahim Khan revealed a monumental secret. Amir and Hassan were half-brothers. Ali was infertile, as evidenced by the fact that his first wife bore him no children, but bore her second husband three daughters. It was Baba who had gotten Sanaubar pregnant, making Hassan his son. Rahim Khan explained that no one but himself, Baba, Ali, and Sanaubar had known about the matter in order to preserve their honor. Hassan never found out. Amir was furious at all of them for keeping the secret. He screamed at Rahim Khan and left the apartment.