So here is a confession. I have a sister and I always look at her body like I was looking at a wall when she was wearing undies or even naked. I never felt jealous of her boyfriends. I never cared. My brother-in-law and I have always been friends, by the way.
Even so, I know that deep in my hippocampus these memories did things to my psychological, those situations have created some light issues and fiction is the only way to "please" these issues.
So... The plot twist is that 'almost' everyone has those small incest issues... I think
Now, if you believe in everything that Sigmund Freud said, the whole of humankind is perverted and declined to incest.