That is disgusting.
I don't if its morally permissible. I'm sure it would hurt they're family if they found out. If that's not a possibility i guess it would be okay if they're using birth control. But birth control can fail, I got pregnant on birth control. Its just creepy especially if they grew up together.
I don't think morality is anywhere near as elusive to define as most people seem to think. There's the sociobiological explanation championed by beckoff and pierce (that it is a suite of behaviours that foster social cohesion) but more realistically for humans, I think it's a matter of "if this action will cause deep existential harm to a being, it's immoral". I don't think it has to be anymore complicated than that. Of course, most people don't agree with me, which makes it apparently subjective, but I don't think it is. Beings generally don't like to suffer, and that's not a subjective claim, that's a fact. So I think moral principles can be based on that objective fact.
Actually no, there are universal principles within a species (excluding exceptional cases such as psychopaths). Jonathan Haidt's extensive surveying of over 23 000 people determined that every person believes that matters of harm and fairness are moral matters (IOW, it's immoral to commit infractions of those natures). There are also 3 other moral domains that about %50 of people think exist, which are purity, respect for authority, and ingroup loyalty. So there are indeed universal principles upon which we can establish objective moral codes due to our biological commonality.
And how is the alpha status transfered from one male to another... through a successful violent challenge, the defeated alpha not only steps down but generally steps right out of the pack, left on its own to eventually die of starvation, for wolves are social beasts which attain a much higher success rate when hunting in groups.
We had one such wolf hanging around our work camp this summer he was long and tall, but quite emaciated and unfearful of human proximity, he was hungry and weak.
@Radu: I definitely agree with your statement that there's a good evolutionary reason why the majority of people find the idea repugnant. It's possible that because inbreeding results in a weaker or less sturdy genetic makeup some form of biological morality (mentioned in someone else's response) makes us react the way we do to the thought of it. I think that plus intensive socialisation probably has a lot to do with why you rarely hear of incest. There have been cultures in the past where incest was not frowned upon, showing that socialisation certainly has a role to play and that it's possible that our biological aversion to it might have kicked in more recently than we'd like to think. imho
Thats raises the question about who decides what's moral and what's not
In ancient Egypt thousands of years ago it was morally accepted for siblings to have sex..they even got married to keep the Royal blood of the ruling family pure..it was accepted during that time..these days it's not accepted.
However marriage between cousins is accepted in Egypt and most of the muslim world these days..it's not accepted in other countries.
The culture and the background has the upper hand in deciding what's moral.
The question should have read ....with each other" Leading from that... why do most of us feel it's repugnant? Is it a cultural no-no and/or something religion has foisted on us? I believe that many traditions and rites contained in religions had their origins in 'common-sense' issues: e.g. circumcision was probably a necessity in the dry barren countries of the Middle East; so also with eating pork which one could imagine was pretty infested with diseases in those years. Sex among blood relations proved to be bad for the gene-pool and resulted in taboos in many communities. Our prime aim as a specie is the survival of the specie...hence inter-sibling sex would be a step backward. I live in a community where the gene pool of Indian/Pakistani Muslims is relatively small; it is noticeable that the incidence of physical/mental defects among their children has increased over the last 50 years or so. (To research it would be interesting but politically challenging) To sum up...the morality of inter-sibling sex is based cultural taboos which have created a psychological barrier/resistance against it.
A child raised without such beliefs would not find sibling sex disgusting. Actually the words gross and disgusting almost always represent a religious attitude based in lack of knowledge. They are words I chose not to use. The incest taboo is a complete social fabrication. Yes the percentage of genetic abnormalities sees an increase hence the common, but NOT universal taboo. But excluding the procreation aspect, if the people were not raised with that taboo, they would not be mentally harmed by their sexual activities. The study cited is very poor science as it has no control group. In order to test a taboo, one would have to subjects of both perspectives.