NOW HERE'S THE STORY of a book that nobody seemed to want, one that may wind up earning as much as Lupe or even more. Last year, when Granada, a British publisher, was approaching every major American publisher with Carolyn Slaughter's first novel, Relations , they could find no takers. The theme - brother-sister incest in Victorian England - turned everybody off. At last, a small, striving house, Mason/Charter, stepped forward with a modest offer and acquired the book. Then things began to happen. Incest became "in" - the sin of choice. In the June 27 issue of Publishers Weekly, Barbara Bannon gave advance reviews to no fewer than three novels with incest themes. NBC did a "Weekend" show on the topic - one and a half hours. The Literary Guild picked Relations up as an alternate selection. A film version was optioned and the script is being written now.Mason/Charter elevated the book to their lead fiction title and are even at this writing negotiating a floor deal with a paperback house, prior to setting the auction date. Interest - belatedly - is running high; Carolyn Slaughter has delivered her second novel to Granada, and she may turn out to be a very hot property indeed. So it would seem that the wages of sin are cash.