In fairness, if the people that are being taken in so to speak allow her to carry on like that, I'm not sure what you could do to stop it. If they're on the dimmer side of a low watt bulb, then I wouldn't have thought any facts that you present would be understood/taken on board for what they actually are.
Also, people get caught up in the physical, one of my docs sees chronic fatigue and unexplained symptom patients. Even when every test has been done and nothing abnormal has been found, parents and patients often still want to pursue the physical rather than the mental because they see the symptoms and how they're effected by them; it's all very real to the family and patient and it can take a lot of work to get in to the psychological side of some of the issues.
Even if this girl is just putting on (and actually not unwell in another way like some of my lot), you could be hard pressed to get your family to understand because they'll see the physical symptoms she has (pain, difficulty walking, whatever...) and want to comfort her. It's probably easier than believing that she's actually just not a very nice person.