Is my sister anorexic? PLEASE HELP ME!!?

I'm very worried about my sister! She has never been a very big eater and both her and I are very thin and in shape (we are runners). Lately, at the age of 13.5, she has began eating even less! At first I thought it was just a phase but when my parents took note I feared somehting more!

She has greatly reduced the amount she eats and denies it whenever I confront her. I ask her why she is eating less and she says she isn't. I tell her she is acting anorexic but she says no. Everytime someone talks to her about eating she gets very defensive and angry.

She has for a few years said that her legs are too big. They aren't. Now, I really think she may have a problem but I can't be for sure! She still eats some things, but hardly any. My mom has threatened to not let her run if she doesn't eat more, but nothing changed. She looks at calorie counts nearly obssessively.

IS THERE A PROBLEM? IS SHE ANOREXIC? WHAT SHOULD I AND MY FAMILY DO?