I'm not an expert on Gothic literature, but Lara Parker teaches English and no doubt knows that incest themes are common in the history of the genre, an offshoot of Romanticism. It may begin with "The Castle of Ontronto," which I've never read. (Byron had an incestous affair with his half sister, and didn't Percy Shelley have some similar situation?) In the 1818 version of "Frankenstein," Elizabeth is Victor's cousin. In "Wuthering Heights," Heathcliff and Cathy grow up almost as siblings. "The Fall of the House of Usher" is generally interpreted as having incestuous themes, but I don't know if that's due to later Freudian criticism or not. Clearly Poe was writing one of the quintessential Gothic stories. There was a suggestion of incest (if I remember correctly) in Roger's affection for Carolyn at an early stage of the series, too. So by including a hint of incest, Lara Parker would be writing in the Gothic tradition, which includes "Dark Shadows."
On the other hand, is a grand-niece all that close a relationship? It used to be fairly common for first cousins to marry in this country. I was just reading a friend's genealogy of her family that came to Maine from England in the early 1600s, and the book (published in the 1800s) says that many of the grandchildren of the immigrant patriarch intermarried. And Franklin Delano Roosevelt married his first cousin, Eleanor.