Are you sure this answer is current and correct? I have been researching this very issue online and the answers I'm finding online do not appear to be as cut and dried as this answer implies. I'm seeing many instances where family member ARE being held liable.
Here is only one of the many statments I found online regarding this subject:
"If there has been a diagnosis of Alzheimer's, a victim of an accident will not stop their suit at the liability limit of the auto insurance. If prior knowledge of incompetence can be found, they can sue. Auto insurance will indicate you knew but did not tell them of the medical diagnosis, and you could be in the liability soup."