kaikeyi wrote: I dont even know someone from our place who suffers the same thing.
Quick question; what do you mean by this? Where are you from?
It's almost impossible, but try not to compare yourself to others. I'm sure there are a lot of things that you have over your sisters; singing for one. Which is amazing! Personally I've always been envious of people who have really beautiful voices. Purging will most likely affect your singing, if not now, probably eventually. I can't say for sure because I'm not a doctor, but it can't be good for your throat.
Next time someone compares you to your sisters, or makes a point about your size, say something. If it's bothering you, then let them know. They deserve to know what jerks they are being. It would make me furious!
And I don't know you, but it's very probable that you aren't fat. Most people with eating disorders think they have too much weight on them. Remember that everyone is a different shape, some peopel look better with more weight on them, some people have big bones (yes it's usually a line, but it's still true). Try to look less at your weight and try to accept yourself as you are. No one judges you half as much as you judge yourself. If you'd still like to lose weight, slowly phase out the bulimia, and start a diet, start exercizing. Try joining a gym with a friend; it's more fun that way and it will give you motivation to go.
After all that, all I can really say is, good luck.