I hate to almost mirror what painted wolf said but it sounds to me like we are very similar in belief, but I do consider myself a Satanist.
I don't believe in Heavan or hell or any other "supernatural" fairy tale lands of any religion that we supposedly go to after death. People generally fear what they don't understand and death is the great unknown so what they do not understand and fear they have to make sense out of no matter how ridiculous the fables are. That fear manifests itself in the creation of elaborate gods/devils and fairy tale lands and whatever else that they need to create and believe in to give themselves some sense of security and an enemy (generally aspects of themselves ) to fight.
IMO, All belief in the afterlife seems to stem from fear, hope, or both- not any real knowledge of what happens after death, none of us have ever spoken to a corpse, no one knows or ever will. To believe or follow a religion and it's precepts whose main reward comes in the hereafter, even when it limits the progress and blinds the person to their animal self, is harmful. These fear based beliefs are an aberration and a practice in self deceit that should be shunned.
"Satanism" as it is popularly called is as individual as the Satanist themselves, there are no "true" bibles, holy books, popes, etc. Lavey, Aquino, and many others are just other human animals with ideas about our existence, but as far as I am concerned none should be "followed" I or you or anyone else is just as qualified to write a "bible" or so called "holy book" as anyone else whether living or dead. Lavey is pretty much the reason most call it "Satanism" yet it could be called just about anything- or nothing.
I've never heard of the marrying of family members, but as I have said before there are no central churches, popes, administrations etc. that could or should have any true control or feel they have a say in what the individual Satanist does and if they tried, they would probably be mocked and shunned anyway and rightfully so. I have to agree with Painted wolf, it sounds just like another "shock" tactic to scare or disgust some people away from it, not much different than the fictional accounts of SRA (Satanic Ritual Abuse) in the 80s and 90s or any other B.S. that has been heaped upon it by the perverse imaginations of the "good" people.