Over at Slate the advice columnist received an email from a man who found out that his wife is really his half-sister. If you don't want to follow the link, the back story is straightforward, the couples' parents were lesbians, and used sperm donors. Recently the man sought out the identity of his biological father at the urging of his wife, because they have three children and she thought it would be important to have that information for them. That is how he found out that they shared the same biological father. Here is the part that has me concerned about realism on the part of the advice columnist:
I don't see how you can keep this information to yourself. She's bound to sense something off in your behavior and you simply can't say, "I'm struggling with father issues." I think you have to sit her down and show you what you've discovered. Then you two should likely seek out a counselor who deals with reproductive technology to help you sort through your emotions.
I don't see why your healthy children should ever be informed of this.
That Dad didn't want to find out who his sperm donor was is a sufficient answer when they get old enough to ask about this.
The coefficient of relatedness of these children's parents is 1/4.
If they ever got genotyped or sequenced and had a pedigree constructed the fact that they're the products of a consanguineous relationship would stick out like a sore thumb.
Even if only one of the siblings inspected their own genome the signs of inbreeding would be clear. This doesn't imply that the parents have to tell their children now about the facts of their genetic origins. That's probably not prudent. But people have to start becoming realistic about the fact that genetic information is going to be widely disseminated and manipulated within the next decade by the general public. In the case of the couple above I think it is probably advisable to tell their offspring the details of their genetic background upon adulthood. Partly that is due to the fact that I have a hard time imagining that this just wouldn't come out in the day to day ubiquity of personal genomics. But partly it is because though the children may seem healthy, it is likely that they suffer some inbreeding depression, which may not manifest immediately, but might put them at higher risk for morbidity later in life. Addendum: This form of incest is genetically equivalent to avunculate marriage. That is, the relatedness of half-siblings is equivalent to that between uncles and nieces.