Friday's 20-vehicle pileup on Highway 401 in Whitby has claimed a fourth victim, a 10-year-old-boy, who police say has died of his injuries in hospital on Sunday.

Ontario Provincial Police confirm the boy is the brother of a 12-year-old who died shortly after the crash.

Their parents, who were travelling in a separate vehicle, remain in hospital.

The OPP are also working to extricate the remains of a man and woman trapped in a vehicle and are in the process of notifying the next of kin.

Police confirmed they were not related to the boys.

The crash happened at about 10:30 p.m. when a loaded tractor-trailer rammed into a group of cars that had slowed down at a construction zone. That collision created a domino-effect of multiple crashes near the busy highway's westbound lanes, just east of Salem Road.

Police are continuing to investigate. So far there's no word on charges.