What mystery is brought up in the final pages of this chapter?

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.