A Blue Mountain family's car overturned after a semi-trailer crashed into the Subaru station wagon on the Hume Highway near the Sutton Forest twin service stations at around 9.35pm.

NSW Ambulance crews arrived to find a man and woman outside the vehicle and their two children trapped inside.

Rescuers were unable to revive the six-year-old boy after he was released from the back seat of the car.

His 11-year-old brother was flown to The Children's Hospital in Westmead in a critical condition, suffering from facial injuries.

The boys' 46-year-old mother was taken to Westmead Hospital suffering head injuries and was released yesterday.

Her husband, 45, was airlifted to the Prince of Wales Hospital at Randwick, suffering spinal injuries.

Police are calling for the truck driver and witnesses to the accident to come forward.

Anyone with information is urged to contact Bowral Police or Crimestoppers on 1800 333 000.