Irene Yaymadjian, PsyD
Sorry to hear that your sister has been diagnosed with Anorexia. Anorexia Nervosa is one of the feeding and eating disorders listed in DSM - 5. One of the criteria for this diagnosis is significantly low body weight. A person suffering from it may have an erroneous body image of being fat and are afraid of gaining weight. Sometimes people suffering from it avoid eating, at other times they use laxatives, vomiting, enemas or diuretics to control and reduce weight even when their body's weight is significantly lower for their body size and height.
Apparently, your sister has been seen by a therapist and or a psychiatrist who have recommended that she goes through rehab. She needs to explore where the erroneous body image has come from and let it go and allow self to be healthy. You can ask guidance from her mental health provider regarding how and in what areas you can be supportive. Hope they have family therapy sessions that you can participate in and let her know you are there for her and you love her just the way she is.
Dr. Claire PSY. D.