Ok, I realise this thread is quite old & has been brought back several times, but I found it while randomly searching for something else, so here goes (long post)...
Weight is DEFINITELY an issue in sororities, whether we like it or not. It may not be what we really want to look for in a sister, but in effect, we want sisters who we are comfortable with representing us, therefore people are sometimes discriminated against when they are have an undesireable weight or look. It is an issue for ALL females, even a woman who is perfectly happy with her own weight (and very few are) has to deal with other women every day who are NOT, so in esscense, it effects EVERYONE.
As a few people have stated, talking to a girl with an eating disorder, reguardless of her weight can be VERY tricky (yes, there are "normal" sized girls who have eating disorders that no one is aware of.) Unfortunately, (or is it fortunately?) I know this from first-hand experience...
...When people notice that I've lost weight, especially if it's another female (the competitive thing), I completely relish it & it makes me want to lose even more weight. When I am questioned ("why aren't you eating?") or criticised ("you need to eat more!"), it only makes me want to eat less. To eating disordered people, all of these type of comments are twisted around in our minds to sound more like, "You're still too fat, you need to lose more."
This is the perspective that today's society has impounded into so many girls' minds. Some people have NO IDEA what can trigger an eating disordered girl into making herself even worse.

And just like compulsive overeaters, bulimics and anorexics KNOW that what they are doing is unhealthy, but it doesn't phase them, because the ultimate goal is "perfection", and they are willing to pay nearly any price for that.
So many different factors account for causing these disorders, but getting down to the real issues is what might ultimately help, if anything (i.e. speaking to a professional). We want so desperately to draw attention to our problems so that others will seem worried (since this ends up being positive reinforcement to a continued downward spiral), yet we don't want to actually get well. It's really very ironic; a rather Catch-22.
The best advice I can offer for someone who has a sorority sister, friend, or loved one with an eating disorder (here I go again, giving advice yet not listening to my own advice) is to talk to her about anything EXCEPT weight and food (school, home, relationships, work, etc), and try to clue into anything negative said, and perhaps you might dig up some roots.
I know what you're thinking- "she's crazy". You're probably right.