An excellent question... and I am sorry to hear of the difficulties your sister has been going through. It isn't easy for her or the family members who are/were close to her.
In answer to your question, a neurologist would be a good place to begin, but a neuropsychologist may be able to give you a better set of answers for your family. The M.D can certainly track the level of damage, determine which specific areas of the brain have been/are being compromised, and lay out a chart of deterioration one may expect with a family member suffering with the diagnosis s/he will assign to your sister; but a neuropsychologist can give the battery of tests that will assess her current level of functioning - both intellectually, and also in other aspects of cognition (i.e., visual/spatial, interpersonal, emotional, common sense, etc.). This may also have some impact with the rest of the family.
In any event, you are on the right track and I wish you well. If you are satisfied with the response, please hit "Accept." That is the only way I can receive credit for my answers. Thank you-