I have a sister who practices dark magic with a group. I recently have uncovered that SHE has been the one stalking and spiritually attacking me for 2 decades. Is there some kind of help for this?

I would seek a Shaman

I'm not sure anyone there can help you but I would contact someone directly at the camp

Although Cassadaga Spiritualist Camp embraces all religions, they do not believe in magik or do any type of practices like that. They are in the religion of Spiritualism, which is a Science, Philosophy, and Religion of the continuity of life, that is it. You should probably find a local Elder practitioner within a group or circle near you that practices that type of thing.