Twin girl stable after horror NSW crash

A seven-year-old girl whose mother and twin brother died in a horror head-on crash on the Hume Highway is recovering in a Sydney hospital.

Her 49-year-old mother Julie Bullock died at the scene of the two-car collision on the highway at Pheasants Nest, south of Sydney, just before 8am while she was driving her children to school on Tuesday.

It's believed the second vehicle left the northbound lanes, crossed over the nature strip, hitting the Bullock's car in the southbound lanes, but police have not confirmed this.

The twins, Hudson and Sienna, both survived the impact and were freed from their Mazda 4WD by emergency workers, but Hudson died later at Liverpool Hospital.

Sienna's condition had improved from critical to stable by Wednesday morning.

Her father Darren was at her bedside at The Children's Hospital at Westmead, the Seven Network reported on Tuesday night.

The driver of the other car, also a 49-year-old woman, was also reportedly stable on Wednesday.

Police investigating the crash have not yet spoken to the woman, police told AAP.