Inspector Gerda Swart said the girl and her family were drinking alcohol when the incident occurred on Monday night.
"The girl, along with her mother, father, brother, sister, uncle and aunt, were having a party at her home in Thornhill, after her parents received their government grants on Monday," Swart said.
The girl fell asleep and awoke to find her uncle carrying her to the outside toilets where he pulled down her shorts and raped her.
"The girl then managed to escape and told her brother what had happened.
"Nobody did anything and the girl went back to sleep... The uncle then slept naked next to her, and undressed her in her sleep again."
Swart said the uncle's wife then found the man on the bed and called the rest of the family.
"The incident was reported to police and the man was arrested."
He would appear in the Hankey Magistrate's Court on Thursday, Swart said.