The thing that attracts/repulses me about this how it tests my small-l liberal impulses. They're not exactly hurting anyone by having the relationship, except the kids (two of the four have special needs, and apparently there's a 50% chance of that with siblings). But what if you and your non-related partners were both carriers of a genetic disease, and there was a 50% chance your children would inherit? Should the state prevent you from having kids, much less getting married?
On a related note, I was looking up autosomal recessive disorders for the purpose of understanding the above, and I came across this nugget of trivia: heterozygote advantage: certain pathological genetic disorders can help protect you from other diseases! Apparently being a sickle-cell anemia carrier brings enhanced protection against malaria, and being a cystic fibrosis carrier may help protect against cholera (not the infection, but the damage it does) and tuberculosis.
And in case you doubt how helpful an extra protection against malaria would be, the July 2007 issue of National Geographic has this to say:
It's endemic to 106 nations, threatening half the world's population. In recent years, the parasite has grown so entrenched and has developed resistance to so many drugs that the most potent strains can scarcely be controlled. This year malaria will strike up to a half billion people. At least a million will die, most of them under age five, the vast majority living in Africa. That's more than twice the annual toll a generation ago.
Compare that to the WHO 2006 data on HIV/AIDS:
...an estimated 39.5 million people are living with HIV. There were 4.3 million new infections in 2006 with 2.8 million (65%) of these occurring in sub-Saharan Africa and important increases in Eastern Europe and Central Asia, where there are some indications that infection rates have risen by more than 50% since 2004. In 2006, 2.9 million people died of AIDS-related illnesses.
HIV/AIDS = 4.3 million infections Malaria = 500 million infections
HIV/AIDS = 2.9 million deaths Malaria = 1 million deaths
And bear in mind, there is no cure for HIV/AIDS, whereas there are plenty of cheap malaria drugs. It can become a lifelong, chronic condition, but it can also be fought off by the immune system alone. So all in all, as horrifying as the HIV/AIDS epidemic is, malaria is worse.