And sadly, now the dog has bitten twice, assuming your father wasn't beating it with a big stick at the time, you need to make a painful decision for the safety of everybody around, including you. You've had him for long enough that if you would have been able to solve his problems, you would have done it by now. And I don't say that lightly. He's still so scared that he's bitten and broken the skin twice, plus any other nips he might have given.
They might have to, but you can't possibly know whether the op and her family have a difficult decision to make or not, based on the very small amount of info provided. And I don't say that lightly either.
Our first rescue dog bit all three of us in his first year and a bit with us. He hasn't bitten us or anyone else in the seven years since and in fact has proved to be a very gentle and well behaved dog.
On two of the occasions he bit us we hadn't recognised the very subtle and tiny stress signals that he was displaying (people say dogs sometimes bite without warning but they always give a signal but sometimes they are so subtle they are almost undetectable unless you know what to look out for) and we had persisted with something he didn't like (eg first time staying with us in a strange hotel). His background (unsocialised/locked in a shed) meant that he found new things very difficult to tolerate. On the third occasion, which was a proper bite that drew blood, not a nip, he was in severe pain and stress having injured himself chasing a bike, and my DH was trying to lift him in to the car to take him to the vets.
So, although one must of course proceed with extreme caution, it is possible to rehabilitate a dog that has bitten once or twice depending on the circumstances and the individual character of the dog concerned, how trainable it is, and whether it's human family can be taught to read and respect it's (sometimes very subtle) signals and react and adapt their behaviour accordingly.