Both Sisters Having Ed, How Do I Help Without Being Trapped?

My younger sister has had a lot of issues over the years - manipulative friends, bad boyfriends and it seems she just gave in to those people. She became bulimic and takes anti-depressants and also sleeping pills.

I thought she had stopped being bulimic until her problems got worse and cut those manipulative people out of her life.
My oldest sister and a year ago broke up with her boyfriendof 8 years. She was the larger of the 3 of us but not too overweight. Now she is just as tiny as my other sister and I'm afraid she has fallen into the same trap as the younger one.

It's hard for me because I know this is going on, and every time the younger sister talks about how fat she is and how much food she is eating and look at how much more food I am eating than you.. it makes ME feel horrible. My oldest sister told me this even one time and now I think that it has taken its toll on her.

I too 4 yrs ago had trouble with this because of my younger sister. I thought I was fat (cause I was way bigger than her) so I started writing down everything I ate, how much, and if it was too much I would throw it up.
I am a healthy girl who exercises constantly because of sports teams, and have realized that way of counting food was ridiculous, so I eat whatever I want now... because really before... I wasn't fat... I was just made to think I was.

The thing is, every time my younger sister says "I'm fat, look how much food I'm eating and you're not etc etc" I get extremely ANGRY with her and yell at her which probably doesn't make her feel better. But it gets annoying especially when I have told her I'm there for her and know about her problems but it doesn't help..

Can anyone help me??? I know this is hard on my 2 sisters.. but it's killing me too....