It was suggested (http://www.amherst.edu/questions/1368/) that, among other criteria, an incestuous couple would have to be infertile in order for their relationship to be considered morally permissible. This is presumably because inbreeding allows for the heightened expression of recessive, deleterious genes. What is the significant difference, however, between an incestuous couple, and a couple of unrelated individuals both of whom have family histories (i.e., genetic predispositions) to chronic illnesses?

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I think the really deep question here is why incestuous relationships seem so morally problematic, quite independently of the child-bearing issues. Here are a couple thoughts.

The case of parent-child incest is clearly the most problematic, even when the child is of age. And here, I think the source of concern is power. It's not that it seems utterly impossible for the child, in such cases, to give truly informed consent, but one might wonder how free (or well informed) that consent could be. It's not unlike, that is to say, supervisor-employee or teacher-student relationships, except, of course, that the parent-child relationship is far more intimate and, as a result, far more is at stake for the child.

What, then, about sibling-sibling relationships? Here, there probably aren't the same kinds of concerns as with parent-child relationships. But, continuing the work-world analogy, it is perhaps worth noting that many companies bar relationships between co-workers as well as between supervisors and employees. Why? Well, the usual reason is that such a relationship can cause all kinds of complications: If it's going well, the partners might form a "block" that undermines the "team" as a whole; if it doesn't go well, well, then that's really a mess. And it seems to me something similar might be said of the case of sibling-sibling incest: The intimate, sexual relationship seems to be in an odd kind of competition with the familial one; if the intimate relationship goes sour, it might well threaten the familial one, and the latter seems like something valuable, too valuable to put at risk. Or, to switch analogies, the conflict here is one of a sort that would be familiar to anyone whose ever considered becoming romantically involved with a good friend...or has somewhat unintentionally found oneself waking up next to said friend the next morning.

You'll note that I've focused here on incestuous relationships. I do very strongly think this is the right thing to talk about. One-off sexual adventures are perhaps another matter, and there are plenty of people who've "experimented" with siblings, often as teenagers, without long-standing ill effects. I'm not saying there's nothing morally problematic about that kind of thing, but there seems to be a morally relevant difference between the one-off cases and "making a habit of it". And when one "makes a habit of it", well, that's a relationship.

That said, to address the question you actually asked: If (say) a married man and woman wish to have children, but know that they have serious genetic predispositions to certain illnesses, then that may well give them a responsibility, at a minimum, to seek genetic counseling. If the risks are serious enough, then there may well be moral questions to be asked about whehter they should have children together. In the case of two family members, I take it that we know the risks are reasonably serious.

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