A seven-year-old girl has survived a horrific crash south of Sydney that killed her mother and twin brother while the family were making their way to school.
Emergency services were called to the Hume Motorway near Wilton, south of Sydney, about 7.50am on Tuesday after two cars were involved in a head-on crash in the southbound lanes, about one kilometre south of the Picton Road turn-off.
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Mother and son killed in horror crash
A woman and her seven-year-old boy were killed in a car crash on the Hume Highway near Wilton, a seven-year-old twin sister is in a stable condition. Vision: Network Ten
Witnesses have reported seeing a Holden Astra travelling north veering across the median strip and hitting a Mazda four-wheel drive travelling south.
The Mazda was being driven by 49-year-old mother Julie Bullock, who was taking her twins Sienna and Hudson, both aged seven, to school at Wollondilly Anglican College.
![Julie Bullock, pictured with her husband Darren, was killed while driving her twins to school.](/web/20170308091346im_/http://www.smh.com.au/content/dam/images/g/u/s/r/3/h/image.related.articleLeadwide.620x349.gusrwv.png/1488920939235.jpg)
Sienna and Hudson Bullock were initially trapped in the vehicle before they were freed.
Hudson was taken to Liverpool Hospital in a critical condition, but died on Tuesday morning, police said.
Sienna was taken to The Children's Hospital at Westmead with head and chest injuries. Police said she was in a critical condition on Tuesday afternoon.
On Tuesday evening, the hospital said her condition was stable.
![Seven-year-old twins Sienna, left, and Hudson were trapped in the wreckage.](/web/20170308091346im_/http://www.smh.com.au/content/dam/images/g/u/s/s/h/8/image.related.articleLeadwide.620x349.gusrwv.png/1488920939235.jpg)
Ms Bullock's husband, Darren, was not in the car and has rushed to his daughter's bedside.
The driver of the Holden Astra, also a woman aged 49, is in hospital in a critical condition.
![Emergency services respond to the crash on the Hume Highway.](/web/20170308091346im_/http://www.smh.com.au/content/dam/images/g/u/s/3/j/2/image.related.articleLeadwide.620x349.gusrwv.png/1488920939235.jpg)
Police are investigating the cause of the crash, which occurred on a section of the motorway where the northbound and southbound 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 northbound lanes of the highway through the centre nature strip and into the southbound lane.
Another witness also said one of the vehicles was travelling north before crossing the median strip into the southbound 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.
The engine from one of the cars was propelled metres away.
Several witnesses stopped and rendered assistance, holding Hudson in their arms until help arrived.
The motorway was closed to southbound traffic for nearly five hours and traffic was diverted onto Picton Road. The motorway re-opened just after 12.30pm.