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Woman, 24, and her brother, 15, died after their Toyota Echo was hit by a Mercedes in Heckenberg smash

Three men allegedly involved in a high-speed, three-car smash-up that killed a woman and her teenage brother in Sydney’s south-west are being sought by New South Wales police.

Acting assistant commissioner Anthony Boyd said the pair’s Toyota Echo was struck head-on by a black Mercedes travelling at “significant speed” on Sadleir Avenue in Heckenberg about 9.30pm on Friday.

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.

Boyd alleged the Mercedes also struck a stationary car before flipping on to 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.”

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.