I agree with Sallynow, I would get them some psychiatric help quickly.
Honestly, though, the subject does tend to come up when your kids are small--they'll say I want to marry daddy, or my brother, or cousin Bobby, etc. and at those moments I have said, "Daddy is married to mommy and some day you'll be married to someone outside of the family. We don't marry family members." (We don't raise and support children that we may someday have sex with them or for them to have sex with each other; we raise them to go out into the world and multiply.)
I know it is true that in the past royal bloodlines did marry their siblings, so don't know if the genetic thing is that big of an issue. To me it is far more about psychology and sociology. We raise our kids in an extroverted way; not to keep them home and producing within one small incestuous framework. I think the taboo is there because we do not raise kids that we may someday 'breed' them to each other or to ourselves. If it is allowed because the kids decide as adults it makes us all wonder how the kids were raised and sets up a model that could lead to abuse in other families--that's how taboos form and that is their function. Our laws often reflect what is culturally taboo--such as incest, to name only one.