How can I forgive my abusive parents and rebuild a healthy relationship?

I would suggest talking to a counselor about this. I'm not in the same situation as you, but I see a therapist sometimes and it can be helpful to put your trust in a neutral person who doesn't know your family personally. It's their job to give you good advice while keeping your problems confidential, even from your own parents. You can start out going alone, or with your sister, then even get your parents involved in the meetings of you choose to. The therapist would act as a mediator to prevent arguments, and help start a healing process that would work for everyone. It's not easy to get help, but doing so is the first brave step to a better future. Good luck