Honestly Pops, I don't think they should ban marriage between relatives. Lord knows I find it vile to imagine that kind of relationship with any of my family members, but it isn't my place to legislate my morality over someone else.
There's nothing we can do about plain old stupidity ya know!

But, that said, there would have to be some kind of regulation on their reproduction. It is cruel to their offspring to allow them to purposefully procreate. I think of it sort of like crime- bear with me I'll get to the point.. We have laws against crimes like murder because they hurt other people. Knowingly procreating in a way that will harm the child is harmful to another person and, therefore, a crime in my mind. Like women who drink alcohol or do drugs while pregnant. It is becoming illegal more and more because it is a crime. The purposeful procreation between close relatives is sort of like that.