I wonder if it is totally made up. There is never any mention of the parents even suspecting anything. I would think as parents they would have at least had the thought that they were a little too close should have crossed their minds at least once. Even if they denied it, didn't they have any friends their age that would have thought something was a little off?
The article sounds like they were shut off from other human contact...even the four years older sister.
It would make me wonder every time I saw a family with the same background of four year older sister, brother a year older than younger sister etc. The comment that it reads like a porno is probably closer to the mark. It leaves a mental scar. Yuck. Thanks so much Balloonslayer...TMI. That is exactly what I needed to start my weekend off with (sarcasm emoticon). Leaving to go scrub my head out with a sterile wirebrush now.