A baby girl has died two days after she was pulled from the wreckage of a car crash in Sydney's southwest that killed her mother and brother.

NSW Police said the three-month-old died in the Children's Hospital Westmead Hospital on Wednesday morning.

Her 34-year-old mother was driving a Mazda SUV on Monday morning when it was involved in a crash with a cement truck.

The woman - named on the Australian Muslim Network Facebook page as Katrina Asma Sila and her two year-old-son Kai Prahastono - died at the scene, while the critically injured baby Ivy was rushed to hospital.

"Sadly Sister Asma Sila and her son returned to their Creator after a tragic car accident," the post says.

The 48-year-old truck driver was not injured.

Police have urged anyone with dashcam vision of the crash or with further information to come forward.