Does my sister have an eating disorder?

My younger sister doesn't look anorexic. She's about 5'8, and probably weights 125 pounds, but her habits make me wonder. Many times she will literally stay in her workout clothes all day, and workout 3 times a day, after every time she eats. And during school season she's on the track team, and even after a track meet/practice she will come home and exercise some more. It probably wouldn't matter to me if she actually ate to make up for how active she is, but her diet isn't that great. She skips breakfast most of the time, and then pretty much just snacks the rest of the day. She never eats full meals. For lunch she'll have something like a banana and cookies, and for dinner she'll have half a greek yogurt with fruit and more cookies or ice cream. I feel like for someone who is that active, she should be eating way more, it just doesn't seem healthy. I mean, I'm a pretty active person, and I eat close to 2,000 calories a day, and I'm 123 @ 5'7.

I don't really know what to think, since she looks pretty normal, and she actually has muscles too. Maybe she just doesn't have a big appetite like me?