Depending on what your relationship with your sister is like, perhaps you could just tell her that when she says you're fat, it hurts you and doesn't help you. Such things just keep undermining your confidence. It would be best to bring it to her attention though because perhaps she isn't realizing the effect it's having on you. Try to convey the seriousness of it as well - it's not just something that's upsetting you, but it might also be negatively influencing your recovery.