Dear S.,
You have several issues which have collided in a very painful way. Perhaps you can separate them in your mind and heart, and deal with them one at a time.
You're two months post-partum; you're sleep-deprived and hormonally challenged. You have less time for yourself than you would like, and are adjusting to nurturing another child. These are significant issues for you, without anyone throwing your emotions off-balance. Forgive yourself first, for whatever failings you might be afraid you have or that may have contributed to this in any way.
Your daughter has constant bombardment from television, internet web sites, friends, school (if she attends public or private school) and billboards. The message is everywhere that anyone who isn't skinny is less desirable and less healthy than the anorexic role models foisted upon the public consciousness. Forgive your daughter for absorbing this toxic message and resolve to keep the lines of communication open. The fact that your daughter feels she can talk to you is a plus; although I agree with others that she needs to develop compassion, and fast!
For compassion, talk to your daughter about any hurtful thing she can recall which someone said to her. Talk about what preceded it and how she felt afterward, even if the offender apologized. Explain that words are our thoughts sent out into the world, and they have power to change things for better or worse. Help her take responsibility for what she says, based on her own reflections of past hurts inflicted upon her.
I agree with others who commented that your daughter needs to respect and appreciate how blessed she is to have two parents and (at least one) sibling with whom to share her upbringing. Explain that family members aren't exchangeable like unwanted gifts, but rather are blessings from God to be cherished for your whole life (even after they pass away). Perhaps in the context of the Haiti earthquake, and the hundreds of impoverished orphans who survived and now face life-threatening challenges... she may learn to count her blessings.
After that "little chat", you could delve deeper into the lifelong commitment that is marriage, and that you're the best and only mother she'll ever have, even if she someday has another nurturer. Draw out her feelings about her new sibling, and offer her encouraging thoughts about the upside of being an older sibling. There are many; offer those which in your judgment will be most useful immediately to restore your daughter's sense of harmony and stability within her home and family.
If you want to broaden your daughter's concepts of "pretty" with regard to body type, you may want to show her Renaissance-era paintings and explain that this was the ideal of beauty at that time, and also art from other cultures which depict beautiful females with actual flesh on their bones. This may help your daughter avoid being trapped in the mindset that she's never thin enough. That mindset can be SO unhealthy!
In closing, I praise you highly for reaching out to a forum like MamaSource, where so much wisdom and warmth are waiting for those who ask. The many people who have already offered their wisdom and righteous indignation have touched on nearly everything I have written. I offer these words in hopes that, by saying it differently or redundantly, the hurt will diminish, you'll forgive yourself and your daughter, and take positive steps to heal the situation without rancor or remorse.
Blessings to you, your new baby and your temporarily insecure young daughter.
Cathi B.