Posted by 
A young brother and sister have been killed after a luxury car allegedly crossed onto the wrong side of the road and hit them in a head-on crash in Sydney's south west.
Police have confirmed the allegedly stolen Mercedes AMG was attempting to overtake before it hit their silver Toyota Echo on Sadleir Avenue in Heckenberg about 9.40pm last night.
The 24-year-old woman and 15-year-old boy in the Toyota died at the scene.
READ MORE: Burger chain stores set alight in Brisbane
9News understands the older sister was picking her younger brother up after he finished a shift working at his new job at Kmart.
Acting Assistant Commissioner Anthony Boyd said from the footage he has seen, it was "very high speed".
"I'm unable to speculate as to exactly what that speed was, that we'll be able to determine when we examine that vehicle further," Boyd said.
CCTV obtained by 9News shows the Mercedes travelling past a house at speed before the crash.
After the first crash, the Mercedes continued on then hit a stationary Toyota Rav4 and flipped on its side.
"The Mercedes AMG has rolled and the three occupants of that vehicle have then escaped from that vehicle and entered a fourth vehicle, which was a VW Golf, and they have been left the area.
"That VW Golf was later stopped by police and an 18-year-old male from that vehicle was arrested at the time."
Boyd said he thought it was abhorrent behaviour.
"I think that nature of the collision itself is totally unacceptable behaviour in the first instance and then to flee the scene and not offer assistance to people in a vehicle.
READ MORE: Wife opens up after alleged coward punch lands husband in hospital
"You may have been able to provide some form of first aid which may have assisted in saving their life."
He said they had strong information and a person of interest.
"All three would be part of the investigation as to what criminal charges others in the vehicle would face would be a subject for the investigation."
The brother and sister are yet to be formally identified.
The two people in the Rav4 were uninjured.
The Volkswagen was seized by police at Ashcroft and the 18-year-old driver was arrested and taken to Liverpool Police Station about 11.30pm.
"He was taken to Liverpool Hospital for mandatory testing."
Police have since released the 18-year-old and are still searching for the occupants of the Mercedes.
"We tried to determine the relationship between those two vehicles and the occupants," Boyd said.
Investigations are ongoing.
