he said one thing that turned out to be true according to his sister. whoop-de-do. and you are feeling guilty because he said one thing that you thought was probably another lie? you didn't scream LIAR at him all you did was think that on past experience he was probably lying.
and I know cataracts can get worse gradually so that you get accustomed to them, but it is still our responsibility, especially when we know we have a medical problem, to ensure that we get it checked/treated regularly and don't put other people in danger by driving when we can't see properly. I'm sorry he was in a car crash but I'm very happy you weren't sitting in the car and I hope no-one else was hurt.