If she is under 18, you need to tell your parents. Honestly, I would tell them anyway. She is engaging in a dangerous behavior and needs all the help and support she can get. Depending on where you live, they may be able to get her into a treatment program without her consent.

The sad reality, though, is that she will need to want to get better. Right now, that doesn't seem to be the case. The right treatment may help that, but it is very hard fighting an eating disorder. 

Your sister is very lucky she has such a loving older sister. I wish you and her the best.