Carl Pistorius "was badly hurt but we are thankful that he is out of danger," family spokeswoman Anneliese Burgess said in a statement.
He had been driving along South Africa's main N1 highway heading to Pretoria last night when his car was hit head-on by another car that had swerved from the opposite side of the expressway into the oncoming traffic, she said.
Carl was acquitted in May last year of culpable homicide and reckless driving over a fatal crash in 2008 in which his vehicle collided with a motorbike driven by a woman who died in hospital a few days later.
Oscar Pistorius is accused of murdering his model girlfriend Reeva Steenkamp in the early hours of Valentine's Day in 2013.
His trial is set to resume on August 7 for closing arguments.