Repentance can manifest itself to affect people's lives in many ways:

"I could almost feel the emptiness in Soraya's womb, like it was a living,
breathing thing." (Hosseini 189). Through what seems to be divine intervention,
Amir's guilt manifests itself as an inability to have a child. Amir hasn't
redeemed himself for his cowardice, and thus isn't able to produce offspring.

"Then late that fall, Farzana gave birth to a stillborn baby girl." (Hosseini 209).
Hassan's repentance comes in the form of his first stillborn child. Repentence not
for his sins (he seemingly dosen't have any), but the sin of his father being not Ali,
but Baba. He also must repent for the many sins of his mother.