Thelma and Kaliff Caulder were waiting to turn left on Highway 87 near Sandhills Road at 4:30 p.m. when they were hit from behind.
Investigators say a car, driven by Ruby McMillan, knocked them into the path of the on-coming tractor-trailer.
McMillan is charged with misdemeanor death by motor vehicle.
Both mother and son were wearing seat belts.
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