Your family will need to hire a lawyer in your state who is familiar with medical guardianship procedures which would allow you to have treatment mandated. Then a family member will need to assume responsibility for your seriously ill sister. No doctor will be able to assume this responsibility, it will be entirely up to the family.
The brain condition that is part of anorexia nervosa
which prevents your sister from seeing how ill she is, is called anosognosia. Please google this and you will see that this can be a feature of many brain disorders, including some strokes. Anosognosia is what makes the management of people with anorexia so challenging. But there is hope. There is also a forum for family members on the web you may wish to consult called "around the dinner table" at http://www.Aroundthedinnertable.Org/ they may prove a life-saving resource for your family and, ultimately, your sister.