I fap to my sister's bully
Why would you ever want to do something like this while thinking of someone who bully's your sister?
I just find it to be sexually arousing.
While it is normal to feel attracted to nice looking people, you shouldn't just be attracted to their outward appearance. Someone who bullies your sister might be physically attractive but she is unkind to your sister so you should be more concerned with her bullying than her appearance.
I can't think of her other than as someone who looks hot, that's all.
It is normal to want to fap to girls you find attractive. Just hope it does not turn you on that she is harming your sister emotionally. If you simply find her to be attractive than that is perfectly fine.