What were the circumstances of Ali's adoption? In the book The Kite Runner

In the book "The Kite Runner," by Khaled Hosseini the circumstances of Ali's adoption were that his parents were killed. Amir is stating that when his father "Baba" was a young boy, his grandfather who was a judge in Kabul had one day brought a five-year old boy named Ali back home.

Ali had lost his parents and was orphaned. His parents were killed by drunk truck drivers so he was adopted and brought to Baba's house. Theoretically, Baba and Ali should have been brothers but the racial difference led to their fates been decided. Ali was a Hazara and not a Pashtun so he was to become a servant. Though childhood playmates, they never view each other as peers or friends.