True about letting everyone know the details of their relationship exposes them to harassment. But maybe they live somewhere where people mind their own business. I'm not so much a 'yuk' person about their sex life as about the bringing a kid into the mix doesn't seem necessary in a world full of children with no families.
I say the grandma will be greatgrandma and the grandson will be dad to the baby to be.
I remember seing noticeably different stats (more like a doubling or trebling), although the risk remains very low (it would give 1%-1.5% instead of 0.5% with your figures.) Don't take my word on that, though.
There's a famous recent case of sister-brother incest in Germany. The couple has 4 children, 2 of them have both physical and mental problems, but it's not clear how much is due to the special nature of the parents' relationship. Since the mother herself is slightly mentally retarded, these problems may be hereditary. Although it's not at all impossible that incest was a multiplying factor in this case.
Incest avoidance is found empirically to be a cultural universal. We generally find incest to be "yucky" because our biology and evolution has programmed us to think it's "yucky" because incest is not advantageous in an evolutionary sense. Children produced from inbreeding (many incestuous relationships lead to inbreeding) really do have a higher rate of congenital birth defects than children produced from typical breeding conditions. Please keep that last sentence in perspective, it's a high enough rate to have helped produce a biological aversion to incest it doesn't mean that inbreeding couples are having tons of babies with congenital birth defects. It's also more advantageous for an individual of any species to have as much diversity in it's genetic code as possible.
Anyways... the above is the basis for my opinion. I think two consenting adults should be allowed to have sex even if they are grandmother and grandson, father and daughter, brother and sister. I also think it's icky and I'm not wrong to think that.
When children come into the picture is where I start having issues. There are a lot of situations where I think it is completely irresponsible to bring a child into the world. The couple in this article are one of those many situations for me. If you don't think of the children before you have them you are already starting off on the wrong foot as a parent. If I were in a relationship that was so far outside the current cultural norm as to be something that every culture on the planet avoided doing I definitely would be thinking of the children and not having them.
There are a number of behavioral traits found in the human population that appear to be universal. Incest avoidance, some form of religion, music or rhythm (drums are pretty much universal) and of course a structured language are universal traits. When the same general behavior is demonstrated in widely separated (by time and/or space) populations it's strong evidence for a genetic component governing the behavior.
Why are people so surprised by this, there is a place in the UK this is common place..........the Rhondda Valley.
Just kidding any valley-ites.
My wife buys those trashy mags and this has been in a few of them recently......but there are other examples of this kind of incest in the past from them......frightening this still happens, especially in this world where we are able to move around easily to spread their gene pool.