Simplify the following text for a Grade 10 native English-speaking person.
Original: The Soviet invasion of Afghanistan sends Amir and his father escaping to the United States , because Amir 's father has been known to curse the Communists . Amir is called back to Afghanistan , by a terminally ill Rahim Kahn , as an adult during the Taliban rule to rescue Hassan 's son Sohrab , when Rahim tells him that he and Hassan are actually brothers because his father secretly had sex with Hassan 's mother .
Simplified: Finally , Rahim Khan tells Amir that the true reason he has called Amir to Pakistan is to go to Kabul to rescue Hassan 's son , Sohrab , from an orphanage .