Why would baba have been ashamed of himself regarding hassan and amir?

When picnicing, amir blurts out that hassan was his brother, sohrab is surprised: Hassan never said he had a brother. Amir explains. At first, sohrab was doesnt understand why people would"Hide it from father and you." Sohrab catches on fast: hassan was a hazora. Neither mention that baba betrayed sahras grandfather.