I remember reading that a Tasmanian virgin was one who could run faster than her brothers...
Honestly, if they are both of consenting age, who gives a stuff...so long as there was no kiddie touching, I couldn't care less. To be honest, waking up next to a female relative would be a relative blessing compared to some of the sins I have committed against my sartorial senses after a night on the turps in years gone by :)
There was an article in New Scientist a few years ago about the increased prevalance of sexual activity between (say) fathers and daughters, who had never met and didn't actually know they were related than there would otherwise (statistically) have been.
The evolutionary reason against "inbreeding" is that you stand a high chance of multiplying any latent defective genetic abnormalities and a low chance of introducing any new 'enhancement' genetic abnormalities.
Anyone who owns a Golden Retriever knows that Hip Dysplasia is an inbred trait. I have spaniels...they are prone to haemophilia...I haven't slept with them yet before you ask...although it does get lonely sometimes, being an RMI holder...