I think this makes me the worst sister on the planet
Why do you feel that way about yourself?
I don't want her to make the whole family suffer for her mistake.
I's normal to not agree with your siblings all of the time, but you should not want to hurt her. It's good that you feel bad for doing so. In the future, you should communicate about any problems that you have.
I'm so shitty.
I think it is normal to feel bad about hurting a sibling. You shouldn't get down on yourself too much. I'm sure they will come to understand your intentions.