I agree with everyone else (in general). The most important thing right now is to just be her loving sisters and lift her up enough to the point where she's ready to get help if she needs it (and if it's not what you think, figure out what it is). And I say if she's up to the job, then you should be, too! Just put enough different body types out there to let all your PNMs know that there's a lot of diversity in this house. After that don't worry about it. What if the PNMs don't make a big deal about until they see you keep glancing at your sister? The last thing you'd want is for everything to go fine until
you bring attention to it!
Remember, girls can be judgemental, but they can be impressionable too--especially if they're trying to prove they'd be a good fit for a sorority. I've never been through a rush, but I know that if I was unsure of how to act to impress a sorority, I'd probably take my cues from them. If you act like everything's fine, they'll probably follow suit. Until then, I'll pray for your sister and whatever condition she might be in.