A seven-year-old girl is fighting for life following a horrific crash south of Sydney that killed her mother and twin brother.

Emergency services were called to the Hume Motorway near Wilton, south of Sydney, at about 7.50am on Tuesday after two cars were involved in a head-on crash in the south-bound lanes, about one kilometre south of the Picton Road turn-off.

Witnesses have reported seeing a Holden Astra travelling north before veering across the median strip and hitting Mazda four-wheel drive travelling south-bound.

A 49-year-old mother driving the Mazda four-wheel drive died at the scene. Her two children were initially trapped in the vehicle before they were freed.

Her seven-year-old son was taken to Liverpool Hospital in a critical condition, but died on Tuesday morning, police said.

The girl was taken to The Children's Hospital at Westmead with head and chest injuries. Police said she remained in a critical condition on Tuesday afternoon.

The driver of the Holden Astra, also a woman aged 49, is in hospital in a critical condition.

Police are investigating the cause of the crash, which occurred on a section of the motorway where the north-bound and south-bound lanes are separated by a grass median strip.

A caller to Sydney's 2GB radio described seeing a car's tyre marks leading from the north-bound lanes of the highway through the centre nature strip and on to the south-bound lane.

Another witness also said one of the vehicles was travelling north before crossing the median strip and onto the south-bound lanes.

"I saw the whole thing and called 000 and waiting till police got there was not a good thing to witness," the woman said.

One of the vehicles rolled onto its roof, and motorists said smoke was coming from the wreckage.

Police vehicles escorted the two ambulances as they took the children to hospital.

The motorway was closed to south-bound traffic for nearly five hours, and traffic was diverted onto Picton Road. The motorway re-opened just after 12.30pm.