What type of doctor is your sister? If she's a PCP, she may not have received much schooling on psychiatric disorders. The reason I say this, is because I mentioned to my PCP that my mother had NPD, and my doctor didn't know what it was. I explained it to her, and she went on to tell me that she has a SIL who behaves the same way. She told me a few things that the SIL had done to her, and it was classic N. I told my doctor to Google it. She really didn't know what NPD was.
This does makes sense to me, because no one is going to visit a PCP for treatment over something an N parent did. Anyone with an N in their family is going to seek out a therapist or psychiatrist. My PCP specializes in internal medicine, so really, that's what she should be studying. I'm sure that people in med school are required to take a few classes in psychiatry, but they probably only touch on NPD. I would think that there would be greater emphasis on things that they would see in their practice, like depression and anxiety. So if your sister is in any area of general medicine, she may not have a true understanding of the disorder, despite her years of training.
Also, your sister being the GC is HUGE. The GC does view the parent differently. In my family, my brother was the GC. I'm certain that he knows things are "off" with his mother, but turns the other cheek, because he has benefited so much from being the GC. I honestly don't think he cares that M has "issues" or not, as long as he gets a check whenever he asks for money. He was never hurt by her the way I was, so he has no interest in knowing what's wrong with her. IMO the child who was the N's victim has a far greater desire to research and learn about the illness. I HAD to know, in order to validate what she did to me. My brother was never abused, so tolerates her delusions, but just doesn't care about the reasons why her behavior is what it is.