Is expiation obligatory for someone who caused a traffic accident that led to the death of his father and another driver, given his certainty that he was not at fault and that the accident resulted from the other driver suddenly cutting into his lane?

If you are certain that the second driver suddenly encroached upon you, and you were unable to avoid the collision, and you were driving within the speed limit, then no expiation is due from you. This is because there is no liability for that which cannot be guarded against.