Attempting to find answers on Wikipedia and Google has proven futile because, of all the search results, it would appear that 95% of them are links to or repostings of Patrick Stübing/Susan Karolewski news articles and completely uneducated wild guess figures from the peanut gallery that posts comments on said articles. Most of my searching seems to result in either people claiming (based on nothing but sheer common "well, everyone knows that") that incest = defects, or people claiming (also based on nothing but sheer common "well, everyone knows that") that "actually, it's no greater than/about the same as/actually significantly less than other factors, like age." I'd like to complain that no one bothered to back their points up with numbers, but the real problem is that everyone backed up their claim with numbers, and everyone's numbers are apparently completely made up and no one's numbers ever seem to agree. One comment I saw claimed that brother/sister incest raises the odds of birth defects "from 0.01% to 0.06%." Another claimed the odds were "ten to fifteen times" greater. The whole thing reminds me of this Freefall strip, with possibly the most noble quote ever for anyone who's been frustrated by a search.
So, would anyone happen to know if there are reliable figures regarding by exactly how much brother/sister incest increases the odds of birth defects? Bonus points if how much other factors (age of the parents, etc.) contribute are also quantified, so they can be compared.
Or, does such data even exist? The most professional-looking (or at least the least "some random guy making a comment"-like) result I found was this Straight Dope article about cousin incest, which threw in this line about siblings near the end almost as an afterthought:
"Let me emphasize we're talking strictly about cousin marriage here. The incest taboo regarding parent-child and sibling unions is still strong in Europe and most other places. Setting aside the issue of exploitation where minor children are concerned, such unions have a much higher risk of "adverse medical outcome"--7 to 31 percent, according to Bennett et al."
Forgive me for being greedy, but despite finally finding a somewhat respectable site with an answer that actually appears to cite a study, I still have unresolved issues with that statement. For one thing, "7 to 31 percent" is a hell of a spread. Were they unable to narrow it down further than that, or does the figure depend on other factors? If the latter, what are the other factors? Also, is this "Bennett et al" study the only one of its kind out there? If there are others, do their figures agree? Basically, is this information reliable?
Finally, to sort of change the subject, a lot of the results I found mention that birth defects can be controlled via proper genetic consultation and counseling, and this Wikipedia article has some pretty good information on exactly what Genetic Counseling is. However, would such a service be available to these characters? Most of my research here seems to indicate that that's something that generally requires a referral, and I'm assuming referrals aren't the easiest thing in the world to get for no reason other than "we haven't done anything yet but we're thinking about it and want to know if it's a good idea." (Maybe I'm wrong?) Also, there's the fact that, to my immediate knowledge, France is currently the only country in the world in which consensual adult incest among immediate relatives like siblings is not illegal, and this whole thing isn't set in France. Is it safe to assume that basically intending to commit a crime would make it harder to actually come forward and admit the details of the situation to the Counselor (or even get the referral?)
Hmm...now that I think about it, that last point would probably lead to all sorts of fun trying to not get caught with the sheer amount of paperwork involved in everything these days, even if the birth defect thing turns out to not be an issue. Even getting the birth certificate without raising suspicions would probably be a trick, and good luck trying to not have the kid in the system...but that's a separate issue I'll try to work out later.