My sister is Borderline anorexic and I don't know how to help?

My little sister is obsessed with her weight. She wont eat anything with sugar or above 100 calories. She also won't eat foo with fat in it. It started about 6 months ago and she has lost almost twenty pounds. She no longer looks like her natural body size and wears half the size she wore 6 months ago. She looks down on everyone else to. That is probably what makes me the most mad. Ever since she found out we were the same size she started wanting to lose weight s it has kind of taken an emotional toll on me as well. It is really sd because se is only thirteen. My parents seem to be ignoring te situations. What can I do or say to make her eat right?