Psychologist specializing in sexual attractions would disagree with you. Ephebophilia is common enough to be considered normal for society and is the most common age related. Ephebophilia is attraction to persons who have gone through puberty (typically 15-21 years old depending on sex and the onset of puberty) but have not physically matured. i.e sexually mature but not physically mature. And yes, the psychology of attraction is an interesting and at times disturbing. Part of it comes with the human brain being hardwired to breed as is the case with all animals. At a base level we exist to procreate. Which is why things that are detrimental to procreation are repulsive to society on a base level. Necrophilia, pedophilia (sexual attraction to prepubescent children), hebephilia (attraction to pubescent children), incest, homosexuality, ect. On the other side you have a basic attraction to things that are seen a contribution to procreation. A pregnant woman is a known breeder, wide hips and large breast psychologically identify a woman more likely to be physically capable of birthing and raising a small child, muscles and height on the male side tend to be identifies of a good protector and provider. Another part of it is age. Mens sexual peak is between the ages of 16-35 whereas with women its between 16-25. Now none of this has anything to do with societal or moral norms. Society rules sometimes contradict psychological norms which is good as human are above the base psychological needs.
I was with my EX for 11 years and she had a PhD in psychology although her focus was working with children, she ended up doing a lot of work on the criminal side and so I picked up a ton from her and reading the stuff she always had laying around. Talking about the quirks of psychology was a common pastime with us.
As far as stepsiblings. It's a bit of a cross. It's a taboo that's not natural. If you break it down a step sibling is an unrelated person who you have lived possibly lived with but who has been a part of your life. No different than a close friend. Sometimes romantic emotions happen. A psychologist would be able to better explain this and if it bothers you, id recommend talking to one. The niece is a bit of a different story as there is a blood relation there so there's going to be a bit of emotional conflict there. But considering relations with a Niece is legal in most of Africa the middle east and southern Asia its obviously kind of a borderline area. Again, I'd recommend seeing a psychologist if it becomes too much of an issue that you either believe you might act on it or it affects your emotional stability.
Games are in interesting and still understudied possible tool for dealing it with. There are studied going on now that are looking at using games as a means of helping people with urges that fall outside of legal and social rules. Look no forth then the amount of loli, incest and landlord/tenant games on here and other game sites.
But again, considering seeing a psychologist. 15 years ago I started seeing one for my PTSD and i know it can be rough especially with the stigma that goes with seeing one but its better to get help if you need it than get taxed beyond your strength and have your life affected negatively about it. Or worse do something that might hurt someone else or land you in jail.
May I jump in? In my opinion, we cannot control how we're wired, just what we do about it. A lot of things squick me, including incest, sexualizing children, and so on. But things just as bad are turn-ons for me--rape, for example. I believe that as long we understand the hard line between fantasy and reality, and stick to the fantasy, we are doing no harm. Calling someone evil for how the person is wired is how the lines get blurred--that's why we keep reading about right-wing Christian fundamentalists being caught with underage prostitutes. To them, the *desire* is evil, so actually doing something about it is just a little further along the line.
I've met people in the kink community who are attracted to children, and who play with other adults who pretend to be children, or read comics, or stories. I admire them for finding outlets for their desires that don't hurt anyone.
So, yeah, rather than games like this being part of the problem, I believe they are part of the solution.
Hope you don't mind my two cents worth.