Worried about my sister and the fact that she might be anorexic. Please help!?

My sister is 5 foot 7 and a little under 120 pounds and wears a size 2. I am worried about her health. She weighed 140 about four or five months ago, but she has lost a lot of weight since. She is 16. She eats very little and confessed to me that she struggles with food and is constantly hungry. Sometimes she eats a lot of junk then eats a tiny amount for a few days afterward. How do I help her? She is very pretty but is very insecure and I think that has something to do with her eating habits. What can I do to help her?