If my dad has been told not to drive anymore but he does drive and has a accident, am I liable also since I knew he was not supposed to be driving?

If you are your father's legal guardian, which would only be the case if he were found to be incompetent and you were appointed his guardian, you could be liable. However, if you are not his legal guardian, then you would only be liable if you let him drive your vehicle, since a vehcicle's owner can be liable for the actions of permitted drives (that is, of someone they let drive the vehicle, as opposed to somone who takes i without permission). If your father is still an independent adult and is driving his own vehicle, you would not be responsible except to the extent you somehow were directly involved in the accident; the law does not make one adult responsible for the actions of another, even if the first has knowledge of some impairment of the second.

(One limited exception to the above: if your father resides with you and you know that he should not drive, it might not be enough to simply not give him permission to drive your car--in that case, you may need to take some affirmative steps to prevent him from taking your vehicle, such as a "Club" or ignition lock, hiding keys and not giving him a spare set, etc. If you made it easy for an impaired driver to take your car, that could be considered negligence.)