In The Kite Runner, what shocking news does Amir learn when Rahim contacts Amir and tells him to go back to Afghanistan?

It is in Chapter 17 that Amir finds out two pieces of
shocking news. Firstly, Rahim Khan tells him about Hassan's death. However, by far the
most shocking piece of news concerns the real father of Hassan. Rahim Khan tells Amir
that Ali, the man Amir had always assumed to be Hassan's father, was actually sterile,
indicating that someone else was responsible for the birth of Hassan. Amir is cleary
shocked by this news that Hassan was actually his
half-brother:

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I felt like a man sliding down a steep cliff,
clutching at shrubs and tangles of brambles and coming up empty-handed. The room was
swooping up and down, swaying side to
side.

Of course, this news
makes Amir's betrayal of Hassan when they were children even worse, as what he did he
did to his brother. Notice how Rahim Khan uses this to force Amir to go to Afghanistan
to rescue Sohrab - Amir cannot ignore the plea for help to rescue his nephew, especially
when he did so little to help Sohrab's father in a similar
situation.