I have to tell you who Harper is to me, because if I don't, this story will not make sense. Harper is my half-sister. We share a father. Her mother is my father's first wife, a woman named Cordelia who died of breast cancer in 1989. My mother is my father's second wife, a woman my father married twenty-two months after Cordelia's death, and a woman Harper has never, in the thirty-six years of my life, forgiven for existing. Harper was eight when my father remarried. I was born eleven months later. Harper decided, at age nine, that my mother was a usurper and that I was the physical evidence of the usurpation. She has spent the subsequent thirty-six years acting accordingly. There have been truces. There have been years of relative calm. There have even been, I think, three or four moments in my life where I believed Harper and I were on the verge of becoming sisters in something other than name. Those moments have always been followed, within weeks, by an incident that made it clear — with total, deliberate clarity — that Harper did not consider me her sister, and was not interested in the process of becoming so.