They can be held responsible for allowing her to get behind the wheel of a car. A car is a weapon in the hands of those who are too old to drive. Some time ago I helped at a day centre. We had an elderly man suffering dementia. His wife used to bring him and collect him later.
One day I was looking out of the window when I saw the wife park the car and got out. Then she set off in the opposite direction towards her house. Now I suppose you could argue that she had every intention of leaving him there, but no.
When I raised the alarm she was brought back. Turned out she was really no much better than he, yet she had been driving him around for a long time.
In that case I doubt if their children would have noticed too much if they were used to them, and if the wife did not go to the surgery much, it could have been missed, but if she accompanied her husband to his appointments, it should have been noticed.
If she had had an accident, would she have been entirely to blame if she killed someone?