Yes, it's a worst case scenario example. However, it shows what some of the consequences of constant inbreeding can be. Underweight, unable to brush their teeth, sexually active children, birth and learning defects...
We are hard wired to find persons with minimal genetics similar to ours attractive. Pheromones are a good example. It's for the survival of our species.
If the incest isn't harming anybody and all parties involved are happy, and preferably not procreating, I don't think they should go to prison. There are numerous cases of GSA where both parties tried to put their feelings aside but could not. It's a strange phenomenon, and I accept it happens. But in a civilised society, we can't be encouraging it. I think it does far more harm than good.