Man arrested over crash that killed brother and sister

Police have arrested a 20-year-old man following the deaths of a woman and her teenage brother in a high-speed, three-car crash in Sydney's southwest

Two other men believed to have been involved in the crash are being sought by NSW Police.

Acting Assistant Commissioner Anthony Boyd said the siblings' Toyota Echo was struck head-on by a black Mercedes travelling at "significant speed" on Sadleir Avenue in Heckenberg about 9.30pm on Friday.

It's understood the 24-year-old woman had collected her 15-year-old brother from work shortly before the crash.

Paramedics treated the pair but both died at the scene.

Officers attached to the Crash Investigation Unit arrested the 20-year-old man about 2pm on Saturday when he attended Liverpool Police Station.

Mr Boyd said the Mercedes also struck a stationary car before flipping onto its side, and the three males inside clambered out and took off.

"Three occupants of the vehicle have then escaped and entered a fourth vehicle, which was a Volkswagen Golf, and they have then left the area," he told reporters earlier on Saturday.

The Volkswagen was found about an hour later and an 18-year-old male driver was arrested and taken to hospital for mandatory blood tests. 

He was later released without charge.

Police were looking for all of those who were in the Mercedes, which had been reported as stolen.

Mr Boyd said the three occupants were not in the VW when it was stopped.

Officers were also trying to determine the relationship between the Mercedes and the Volkswagen, which were travelling in close proximity before the Mercedes moved to the wrong side of the road and collided with the Toyota.

Witnesses told police the men, described as young adults, fled the scene without checking on the occupants of the Toyota.

"The nature of the collision itself is totally unacceptable behaviour in the first instance, and then to flee the scene and not offer help to people in a vehicle .... you may have been able to provide some form of assistance which may have assisted in saving their lives," Mr Boyd said.

One witness told ABC News she and others tried to help but "it was very bad".

Police warned the occupants of the Mercedes to present themselves before they were caught.

"Come and talk to us, assist us with our inquiries," Mr Boyd said. 

"You will be able to give us your explanation, your version of events, before we catch up with you."

He said footage of the accident he had seen so far indicated speed was a significant factor.

"At this stage, in the early stages of the investigation, it is believed that high-speed is a contributing factor for the collision."

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