Since I started writing the "SWMS" or Sex with my sister series back in 2005, I figured I should answer you.
My story started off as me playing off one of my fantasies in a safe and mature manner without involving actual family members. This came from a deeply repressed incident, that I had blacked out, where I was forced to have sex with my sister when I was very young by my two older sisters... yeah that shit sticks with you. When I recalled the incident (a few years ago) I spent a great deal of time rereading my stories and thinking things over. Now, in retrospect, I wish that I had sat down before writing and discussed the incident with the family I had chosen to cut off from contact years later. No, they weren't the type of people you would openly talk to, but maybe it might've saved our relationships as they existed at the time.
As for the subject, My characters, initially in that book, find each other, because of a situation that turns them on (a movie) and they begin exploring mainly because they feel safe with each other. The partners see each other as someone they can trust. A person they've known forever. That trust is easy to play on and set up a relationship with.
Now you may notice that my profile here is kind of empty. Yeah. Deleted my stories from the site and will be putting them back after MAJOR revisions.
Other stories I've done are like "Accidental Incest" where a set of twins (brother and sister) are basically forced to have sex by a series of earth quakes that start while they're in a cave exploring. The cave begins to crumble, and the shaking basically knocks them down and forces them into each other... from there they figure out what to do and how to deal with it.
Okay so then there's the more recent series, High school troubles, where the main characters hook up after the younger sister confesses an attraction to her brother. Yes, they hook up, but mainly so that they can equalize each others love lives without jumping into someone else's bed... too bad it doesn't work out too well and it ends up exploring rape, accidental incest, and other scenarios.
Again, these stories are all back in the redo bin of my computer, and i'm trying to write them to be realistic over all.
As for why are they sought after... well, i'm no psychologist, and a number of people have told me (one being a former roommate) that they don't like them. It's fine. Don't worry, I don't get angry over that.
As for human sexuality, as I have began coming to terms with my own over many years, I can only say that I'm no expert, somethings turn people on, others don't. Former roommate included on that one. What turned him on (cub porn, my wife caught him in the living room jacking off to it one day, and he kept doing it regardless if my wife was there or not.) does not turn me on and doesn't turn my wife on. I like the stories of exploring sexuality and first times, and my wife likes Male on male themes.
So I hope I've helped in some way to assist you in your understanding.