I am sorry to hear about your situation. In my opinion, you are using the excuse of been overweight for your bullying. Yet I am also overweight but while I have faced bullies in the past, my weight has never being an issue with weight, mainly because I am confident with my weight.
Now don't get me wrong, I am not saying this to put you down but help you. Bullies unfortunately are like predators and as such, they can smell weaknesses in others (or what they percieve as weaknesses). As you obviously don't hold yourself in a high regard, thinking that there is something wrong with you, bullies will sense this and see you as a target.
So what will I recommend? First of all, work on improving your self-esteem. I'm sure you have many good points and from your profile picture, I don't see you as being particularly fat or overweight. Anyhow like most females, you don't like your bodily image (I say that for if I had a $ for every female I have come across who dislikes the way she looks, I'd be a Millionaire)!
Hence try and build up your self-image, getting your esteem and worth from what really counts, who you are and not how you look (though as I said I think you look quite nice). Next stand up to the bullies in your life, be it siblings or others around you. You teach people how to treat you so when you start acting from position worth, 90% if not all of your bullies (who are emotionally stunted people anyhow) will cower away or simply be nice to you.
Hope this helps.
Author of Beat The Bully; A Guide To Dealing With Adult Bullying