In this case, being Dickon's sister, and especially their parentage, made those "dark secrets" rather strong. I'm reminded of the basic premise of School, Love, & Friends (the Patreon-safe version of School, Love, & Cousins), suddenly had the MC sharing a bed with his "childhood friend," and once they started a relationship, they had to keep it secret because people would judge "childhood friends," being involved with each other. Those made more sense when they were cousins, but 18-year-old childhood friends, not so much. Sometimes the conversion to Pateron-safe can strain a game to the breaking point.
As far as why the prevalence of incest stories? Other than the basic taboo aspect, it might come from the current porn industry. I see these "step-sibling/step-cousin/step-parent" videos as a lazy shortcut. It allows them to film in one house without having to come up with a reason why these characters are living together.