The risk for first-cousin offspring is only marginally higher than for people generally - comparable to that for the offspring of mothers over (what was the exact figure? Somewhere in the 30s. Pretty low in the 30s actually, IIRC.) For siblings, it is of course also lower than for parent-child relationships, but higher than for cousins.
Wouldn't the rate of risk be higher for siblings than for parent-children, considering siblings have the same genetic material, and a parent only has half of the child's?
Obviously this assumes that these siblings were not themselves descended from incestuous parents. The risks are sort of cumulative. European High Nobility is a small set of families that have been intermarrying for centuries. Small wonder there haven't been any non-deformed people born to them for a while. [/slight exaggeration]
Sexual experimentation among siblings soon after puberty is surprisingly common, especially in large and/or relatively isolated families, and usually ends pretty quickly as well. (I know a story about it from my own family. But I don't think the people in question know that I know.) In and of itself, I wouldn't think that there's anything wrong with people as do that, as long as there isn't a huge age difference. Now, if it goes on for years... something may be amiss.
There's also the scientifically demonstratable bit about how siblings who weren't raised as siblings tend to be sexually attracted to each other to an unusual degree.
Weren't two of your grandparents twins?
