The nature of a virtual novel means some things are going to be decided by the author, not the player. You are being forced into a relationship with Sis if you want to or not, because that is how the story goes. The situation with Shortcake is the same thing. That is the narrative the author wants to tell. Some things are out of the players hands. What if in DMD, you wanted to focus on Georgina and not do anything with D? Well, you are forced to go through all of that right? This isn't a sandbox game, it is a narrative. You get to control the side stories, the author controls the main story and it is clear the main story involves Sis and Shortcake.