Underweight sister won't eat (anorexia)?

I have a little sister who is 14, and I can't get her to eat. She would only eat once a day and it would either be an apple or celery. Almost every single person in my family is anorexic except for my dad, and my mom died from anorexia 2 years ago. I was disappointed in her when she felt sick at school and I had to take her to the doctor. Shes already been to the doctor twice in a row, and the doctor only prescribed her some vitamin pills. Shes 78 pounds and shes 14, considering the fact she told me she wanted to look like Nicole Richie. She was eating 3 days ago when we had fruit salad for dinner and she stopped eating and keeps forcing herself to throw up because she tells me that shes gonna gain another pound. I don't want my little sister to end up like my mom. What should I do now?