I read your blog today, and am familiar with what you're going through. I don't know if this will help, but a few things:
1. Treat your emotional addiction to food like an alcoholic treats liquor. And then deal with it one day - one moment - one decision at a time. Get through today. Tonight. The next few hours. Congratulate yourself for the good that you do and keep moving forward.
2. Think about the difference between need and want and let the need drive you.
3. Give yourself permission to start again. (A new month starts today, but there will always be a reason to start again...)
4. Don't compare yourself to others. Not to your sister, not to anyone. No good can come of it. The successes of those around you should be inspiring and joyful. Jealousy is a useless emotion. What does it get you?
5. There is a team here on Sparks - Basic Monthly Challenge it is called. This month's challenge starts on Monday...it seems to me that it is a great way to keep yourself accountable and moving forward in small, do-able chunks. I don't know how to link to it from here...but you can link to it from my page.
So, maybe I've overstepped...but it sounds like you are on the brink and it could go either way. Don't worry about tomorrow. For now, for tonight, don't do it. And, I know you don't know me....but if you want to talk, feel free to contact me.