What would your reaction be if one of your sisters came and told you this...

[Corsulian]

Be supportive and walk her to the student counseling center. Then wait in the waiting room while she speaks to a professional.

That is virtually the only correct answer to this situation. The chapter is not equipped to solve this problem - it is only equipped to support the sister.

[LightBulb]

Originally Posted by Corsulian

Be supportive and walk her to the student counseling center. Then wait in the waiting room while she speaks to a professional.

That's good advice. It'll probably mean a lot to her that you went with her and supported her.

And now / in the future, when you compliment her, don't let it be based on looks. Don't be like, "You look so cute in that dress!" whether or not you think she's gained/lost/maintained weight. Compliment her on other things (cleverness, creativity, etc.).