Katrina Silva was driving on the Hume Motorway at Menangle Park when she attempted a U-turn and collided with a cement truck on Monday morning.
Both Silva and her two-year-old son died at the scene, while her baby daughter was taken to Westmead Children’s Hospital.
First responders earlier stated it was a miracle anyone had survived the wreckage, from “such a tragic, high-speed incident”.
Despite doctors’ efforts over the last two days, the newborn died in hospital this morning, police confirmed.
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The truck driver was not injured and has been taken for mandatory testing.
Police are investigating the cause of the crash and will prepare a report for the Coroner.
Anyone with dashcam footage of the incident is urged to come forward.