Is what happened to my son considered accidental killing on his part, after his wife and daughter died in a car accident following his wife's insistence on continuing the journey?

If the driver took all necessary safety precautions, inspected the car, adhered to traffic laws, and was fully conscious, then an accident that occurred and which he could not avoid does not necessitate expiation (kaffarah) or blood money (diyah). However, if he neglected any of these precautions, he is then liable for what occurred and it is considered akin to involuntary manslaughter (qatl al-khata'), obligating him to pay the expiation for killing, and obligating his 'aqilah (male relatives on the father's side) to pay the blood money.