Two girls were raped and one of them strangled in Lamontville in Durban.
The second victim survived after she managed to flee from the suspect, aged 20, who is also her brother.
The incident on Saturday night has left the community of Lamontville angry and shocked.
They described the incident as the "most terrifying one ever in the history of the area".
Thabisile Sithole, 13, and her 13-year-old best friend, whose name is known to Sowetan but cannot be revealed because she is a rape victim, had been playing together at about 9pm when the suspect allegedly raped them.
Thabisile's cousin Philile rushed to the scene after hearing noises coming from the flat upstairs where the incident happened.
She said she found the child's naked body lying dead on the floor of the bedroom with her tongue hanging out.
"I screamed for help, she was just lying there. Her tongue had turned green.
"I tried to wake her up, but discovered that she was dead," said the distraught cousin.
It is believed that the suspect, who was the only one at home with the girls at the time, raped both of them.
He strangled Thabisile and then assaulted his sister and stabbed her in the back.
But fortunately, she managed to flee and alerted a neighbour.
Philile said that Thabisile's friend was bleeding from her genitals which showed that she has been raped as well.
The suspect tried to flee but the community apprehended him and alerted the police.
The Sithole family described their dead child as an energetic little girl, full of life, who loved school very much.
Her distraught mother Sebenzile said her child's killer was a well-behaved young man, who always greeted her.
"I still don't believe that this boy who grew up in front of my eyes me could do something this evil.
"I just wish to move away from this area. The thought of waking up every morning and seeing the house where my child was brutally murdered is painful," she said.
The suspect's sister is reportedly in a stable condition in hospital.
Police spokesperson Mhlonishwa Khawula said "a case of murder and two cases of rape are being investigated".