Am I a bad sister? What should I do?

[Tracy]

Stop calling her fat. Even if she is overweight what your doing is bullying by calling her names.

stick to be being sensitive but firm.
explain to her what you said to us about how if she doesn't do well in school and is gaining more weight then there is not a big chance of a career for her. The fact you are trying to help is good though so well done for doing that. Sometimes you need to be blunt.

you could just be like you either lose weight and be the person you want to be or don't do anything about it and stay unhappy while everyone else lives there life.