You run the same risk of mutation regardless of who you sleep with, seeing as mutations are generally external environment based (radiation, chemical, etc.), and so the risk of mutations tend to be geography based, not familial. Sleep with your next door neighbour who is in the same pile of dioxins as you, and it is no different than sleeping with your sister where mutations are concerned.
Now, genes are a different story, but still, recessive alleles are recessive alleles. It doesn't matter who you sleep with. It is not as if your sister would have a more virulent strain of it than Jane Doe from the streets. There is a slight chance if it being higher in your sister than a random person off the streets, but only *if* that recessive allele already run through your family. In a family with no recessive alleles, then it is actually *safer* to sleep with your sister.
The real danger of sleeping with someone of a similar genetic makeup is that your progeny's body's defences tend to be wired to be resistant to what you are resistant to. That means that if something new comes along, the lot of you will cop it. That is what is happening with certain breeds of dogs and other animals, some of which the greenies are so hellbent on preserving. They are, as usual, opening up a can of worms because of their stupidity and short-sightedness, but that is another story. Random differences/mutations in genetic make-up is what allows the species as a whole to survive nature's calamities like the Black Plague.
To recap: Child-abuse is an argument based on emotions, not facts. It is just someone trying to tug on the heartstrings in order to win an argument and has no basis in medical or scientific fact.
