I'm no psychologist... but here is my theory.
I read these stories as well, in fact... I ONLY read brother/sister incest when I visit literotica.
I find it's safe for me to read these since I have no brother and therefore cannot 'fall into temptation'.
-- They are forbidden but very unlikely to make me go insane.
I believe that is the danger with these fantasies. For example... if you watch kiddie porn (which I don't advise anyone to do), your mind becomes weakened by the possibility of acting out what you see (since we are very curious creatures) since there are children everywhere... and they could be easily accessed, one could easily lose their mind and 'fall into temptation'. Same goes with incest stories. We are often afraid of what may happen to us if we let ourselves be pulled into things... and therefore we stay away from them. However, there are times that for some reason, we end up being pulled into them... and then we're caught in their trap. They become an obsession that either we continue filling with fantasies... or make them a reality.
When we read, we often place ourselves in the main character's shoes. We see ourselves with our family members. That is also a reason we tend to stay away... because next time we see our mother or father, we go back to the story. It's like alchohol, I believe. The first taste is disgusting... but then you slowly acquire the taste, but if you drink too much... you can become an alchoholic.
Same thing when you've HAD this desire in the back of your head... but realize that it may never be. These stories either keep you sane or give you the courage to act out the plot.
This person is either mentally strong... or has a secret crush on mom. I mean... sometimes moms are hot... it doesn't mean he'll act out his desires. When you're close to mom or dad in the way that you're supposed to, you don't even dare think about them like this (if you were raised appropriately). So either he's not that close to mom... or he may actually have a thing for her.
Well... but of course, I'm just sharing my thoughts.
We love danger... but we all know we have to stay as safest possible. That is why I dare not read sister/sister, daughter/mother, daughter/father stories.... and rather stay with brother/sister stories. If I have no brother, where lies the danger?