Is there any way to find out what state the hosting servers are located in? It's very possible that the initial argument still holds if there are state or even local laws prohibiting incest.
This PDF is a good resource on state incest statutes and the severity of the crime. http://www.ndaa.org/pdf/criminal_incest%20chart%20_2010.pdf
I find it curious that every state considers incest a crime when there is no federal law regarding it, so really the argument should be if discussion of what is considered a crime should be banned according to local laws or federal laws.