because they're real siblings. Step-siblings juuuust doesn't count to get the wincest factor. I don't understand it since the entire point is that fetish aspect but I don't see how new step-siblings who aren't related by blood could possibly do it for anyone if they're really looking for wincest as daydream fodder or whatever. It's actually just two strangers meeting and living with each other. Most shows just don't have the balls to go full-on incest (and most of the shows that do keep a pretty low profile, like Koi Kaze. You're just not gonna make it big with a full-incest show because you'll weird out too much of the audience. Don't even know how Oreimo managed, cute girls and a really slow to develop relationship I guess).