Who can sue whom and why? Anyone who was a driver or passenger who was injured can make a claim against the at fault driver for loosing control of his vehicle. Since it was raining at the time of the accident, there may be a claim against the highway department for failure to properly maintain the highway assuming there were numerous accidents at that particular location and/or something was wrong with the highway which caused hydroplaning of vehicles. Since the driver was 85 years old, the family who allowed him to drive may be liable if they knew that he was an incompetent driver and allowed him to drive anyway.
Of course, if there were not enough insurance coverage on the at fault driver, then claims could be made against the insurance companies of the injured parties for uninsured motorist and medical pay benefits.