Rahat is Nusrat's brother's son or Nusrat's Nephew
Rahat Fatheh Ali Khan is Nusrats Nephew. This is because Rahat
Fateh Ali Khan is the son of Farrukh Fateh Ali Khan and Farrukh
Fateh Ali Khan is Nusrats younger brother. There is some
possiblities that Nusrat may have two other nephews Rizwan &
Mauzzam. I do not know how they are connected because these two may
come from one of Nusrats four older sisters. I do not know about
this but he is a uncle to may, who aren't well known to the
world.
Nusrat Fateh Ali Khan does have a daughter called Nida.