Second driver in crash that killed a mum and son has also been named as a mum of twins.

A tragic coincidence has been revealed in the crash that killed a mother and son and left the little boy’s twin sister, Sienna, fighting for her life on Tuesday.

The other driver involved in the crash that ended the lives of Julie Bullock, 49, her seven-year-old son Hudson also has twins who attend the same school, Nine News reports.

Ms Bullock was driving her children to Wollondilly Anglican School, in Sydney’s south, when her car collided head-on with another which crossed onto the wrong side of the Hume Highway about 8am.

Ms Bullock was killed on impact, and Hudson died shortly after the crash. Sienna was taken to Children’s Hospital at Westmead where she is now in a stable condition.

The second driver Tina Vibert, 49, who also reportedly  has twins attending Wollondilly Anglican School, remains in a serious condition at Liverpool Hospital.

Ms Vibert has not yet been well enough to speak to police and investigators are yet to determine the cause of the car crash.

Father and husband Darren Bullock has been by his daughter's side since the crash, Wollondilly Anglican School headmaster Stuart Quarmby told Nine News.

"He is obviously deeply affected he is sitting beside the remaining part of his family which is his little girl. He is praying for her and looking after her," Dr Quarmby said.

A GoFundMe page has been set up to assist Sienna and Darren - surpassing its $25,000 goal in just one day. It has so far raised more than $31,000.