I am going to say immoral because there is no such thing as 100% Safe Sex, they are still choosing to take the risk and have that slim and very rare chance of creating a child between them.

I can’t ascribe a moral/amoral label to incest because it goes against our genetic programming to mate with close relatives. Therefore it isn’t a subject that’s “moral” or not, it’s a subject that’s beyond simple morality and into an area of being human.

From what I've read, it's about keeping order in family and society. If sons can take mothers and sisters to bed, they would dethrone their fathers and brothers. Fathers would murder their sons. If daughters can have sex with their fathers, it would misplace mothers. Mothers would kill their daughters.

So, we now have extended rules that first cousins are not to have sex or marry each other. Same reason... families would end off killing themselves over affairs. In short, incest, or the possibility of incest, leads to murder in the family.

Instances where incest happens and it is accepted is in tribes. Males have children with many women. Women have children with many men. There are lap overs in parenthood as children born into the tribe are the tribe's children mainly under the care of their birth mother and her close siblings. Different system, so different rules.

Incest also happens in the animal kingdom. Animals don't have the intelligence of humans, so they may not get jealous or have a taboo thought of incest. I have heard of male lions killing off the offspring of previous males when they mate with a new female. I think that is more of a lineage dominance factor though. I've heard of dogs of the same litter mating with each other. They don't have the rules humans do.

How did Cain and Abel, the only two offspring of Adam and Eve, produce children? - Sex with their mother? Or if Adam and Eve did have other female children who are not mentioned, they would have been full sisters of Cain and Abel. Enuff said!

I think that the general aversion to incest goes back well before Christianity, so it's not just based on a religious taboo. It also seems to be very rare among most higher animals, certainly the primates, although there are exceptions, and sexual curiosity (I.e. exploratory, rather than penetrative sex) often occurs in animal family groups.
When a female animal comes into heat, and emits pheromones, it seems that generally other siblings in the group are not attracted by those pheromones, so there's a biological restriction against it.
One of the main human objections to it is that we generally have a different sort of relationship with close relatives to the sort of relationship that leads to a 'normal' sexual attraction.
That's why the cases where siblings have been raised apart and then happen to meet, not knowing that they are related, and fall in love and have sex, are so interesting, and so hard to condemn. It basically goes back to my point that the relationship you have with the person that you know is your brother or sister, and that you were raised with, is not the sort of relationship; that usually leads to sex.
But, with all the caveats and qualifications that have been mentioned by the poster and others (safe, consensual, adult, etc), then there's no a priori reason why it should be wrong.

If they're consenting. that means they are adults (because minors cannot, by definition, consent to sex). If that's the case and there's no possibility of offspring, I really don't care, personally. None of my business.

I don't like the question as written, either. Because, frankly, if we used the words unethical/ethical instead, it could be either. But the probability is so high that the privilege would be abused, I have to say that it's immoral about 99.99% of the time.

This question presupposes that a sexual relationship between consenting adults is always good, and is framed to minimize or remove most of the obvious problems with incestuous relations: power dynamics, possible genetic repercussions, etc. But incest is always a slippery slope, and not for genetic reasons as much as social reasons.

When people are related to each other, they already HAVE a familial relationship. Introducing sex into that relationship is likely to have significant negative consequences, and they cannot all be foreseen ahead of time. Woody Allen chose to become sexually involved with his then-wife's adopted daughter Soon Yi, who later became his wife. They weren't genetically related, and they are still married, but that relationship sundered his family, and ALL the other relationships in the family unit were affected. Woody's choice, to sexualize a relationship with a young woman who once thought of him as a father figure, is a selfish choice, and ignores the impact on the greater family constellation.

I know the post-writer tried to avoid the obvious father-daughter power discrepancy in my example, but the fact is, all relationships have power dynamics, and they can't be ignored. A pair of siblings shares a wider family together; will they brazenly display their affection for each other to their parents? and other siblings? or keep it a secret? Neither is a good choice.

No relationship happens in a vaccuum, and adding sex to a sibling relationship is almost always going to be the wrong thing to do.

Having said that, imagine two ageing siblings living together, whose parents and other siblings have died. Perhaps one of them is disabled, or crippled by poverty. They love each other, and always have. No one will be harmed if they decide to become lovers. But no one need know, either. Both moral, and ethical, standards upheld. This falls into the 00.01% of cases mentioned above.

Even if you were to remove the procreation from the equation, it's still a question of health, but rather than sexual health, it becomes one of mental health.

Remove the word "moral" from your question and ask yourself what is the evolutionary mechanisms that allows this to happen and also creates a social taboo. The urge to procreate is the most fundamental of all living instincts, but there are also a whole host of other instincts that curb that urge which are designed for us to pick the best mating partner to pass on our genes. Our evolutionary minds have been designed to reject familial relations at adolescence, unless there are no other options, because the first directive of reproduction supercedes.

History is rife of examples of incestual sibling relationships, and most of them show signs of some forms of mental and social dysfunction.

Society has certain rules be you a religious or not. There are many religions, I'm sure that would welcome the practice of incest. I like to consider myself a liberal, live and let live, blah blah, but I also work in family law. The cases I have seen in 35 years with incest somehow end up to be more one-sided. Again, with the male convincing the female the it's ok. In fact look at all these people on this website that agree with me. I'm sorry but I disagree. If the only person you can find to form a loving sexual relationship with is in your immediate family, you might want to seek counseling.

In most (virtually all) contexts, it's immoral because it's harmful. One (or sometimes both) parties are too young to give informed consent and/or there's a dysfunctional power balance in place.

I have no issue in theory with, say, two adult siblings shacking up of their own free will. Yes there are genetic risks with any offspring, but not nearly so much as is often supposed and less than some of the genetic risks we generally don't second-guess in more conventional relationships. Yes, I have grave reservations about crossing certain lines even in adulthood, and wonder what younger dysfunction was present to open minds to this sort of relationship. But in that limited circumstance I don't see a clear harm in not prohibiting it.

I cannot see a parent / child coupling ever being harmless, even with an adult child. There, we're back to the psychological power dynamic imbalance again. Clear interpersonal boundaries are essential. Some configurations just aren't defensible or rationalizable, ever.

That guy sucked you in with a bullshit question. The god of Abraham, on more than one occasion created situations where incest was necessary for the human race to survive. I would turn the table and ask him to show anywhere that incest is promoted more than the "inspired word of god".

So, if we're talking about completely consensual sex between adults, such as siblings and even parental and ADULT offspring, it's not immoral at all. I don't have a sister, but I once lost a giirlfriend to her brother and thought that must be the perfect love ... brotherly/sisterly love combined with passionate sexual love. I also knew two boys in high school who allegedly got their first sexual experiences from their sisters. I'm a Cyrenaic hedonist and believe that ANY relationship between ANY two or more consenting adults, for whatever purpose - sex, procreation, companionship ... whether or not an exchange of money or goods is a part of that contract, is their business and not mine, the law's, or anyone else's. Even the most vehemently conservative extremists of every religion are as subject to DNA's draw to pseudo-procreative acts as anyone else. You can't escape it and nobody has ever given me a good reason to want to escape it.

Far be it from me to judge what 2 consenting adults do.

In this exact circumstance I'm going to have to go with moral. The only reason why is because it is a scenario pertaining to two consenting adults practicing safe sex. There's no real health issue, no taking advantage of one party or another and no exploitation that I see. And my personal view on sex in general is that if it's between two consenting adults then it's not really any of my business.

An alternative thought exercise would be if two siblings were in love and wanted to be with each other. They decide to take permanent precautions and get both sets of tubes tied and decide that if they want kids they would opt for adoption. Societal norms aside, would this be a morally acceptable situation?

I do not see it as being in the realm of moral choice.
Nature generally makes such unions less favored among a realm of choices.
To the extent that it does not, it is allowed.
Social norms generally amplify natural tendencies.
As I am adopted, I may have already had sex with or declined to have sex with a sibling. Nature does not care and mostly I do not either.

Mostly this has to do with power balance in a relationship. Obviously, mileage may vary depending on the two people in question. In order for it to be consensual, both parties have to be of proper age, but that's just where the consent discussion starts. There could be a lot of other things going on. Perhaps one is manipulating the other, maybe one is older, or can use guilt over a family situation, or one is struggling and the other has resources to spare. Even if there were nothing biologically wrong with the situation, there are way too many things that could cause an imbalance of power, which is the root of virtually all abuse cases.

I have never heard a case where incest has truly been consentual. No coercion, no power dynamics, bit two adults being intimate without any shady behavior. I don't believe it's possible, especially considering the biological imperative that programs us to view close family as non-sexual only. I think those who some how view close family as sexual have a wiring issue in the brain. If there was an existing case where it was close relatives, started in adulthood and was completely emotionally healthy, maybe that would be ok. I just don't see that existing in real life.

Incest is taboo. safe sex between consenting adults, is not taboo. the taboo real and limiting for obvious evolutionary reasons. this day in age, with muslim fathers either gathering their sons and going on "rape adventures" seeking out anything small enough to hold down. or if there are no sons, simply raping their own daughters, nightly or if you prefer, selling their daughters at age 6 typically to an uncle or cousin, that goes to reinforce the taboo. if you take a look at the effects of imbred babies, you can go down the list and mark off one for one, those characteristics and your typical muslim. and im not simply being bigoted. look it up, in the majority of muslim societies, upwards of 70 percent of all newborns are born with birth defects, typically developmental, yet often physical, directly associated with being imbred. its just the facts.

I recommend, you read Theodore Sturgeon's short story:"" If all men are brothers, would you let your sister marry one?

It is science fiction, so Mr. Sturgeon cleverly creates a margin of safety for his reader, sensitive or not. But I will tell you that is it about love.

I read through many of the comments and I don't understand why so many people are discussing relationships that are part of the question. The question mentions "brother/sister", "safe sex" and "consensual". I think in this case it is moral. It don't ask if we think it is strange or not. It also does not mention if the people are adult, but if not I think this would be no more immoral than two unrelated under aged people have sex . And the morality of that would be another question. A parent/kid sexual relationship start to get tricky. I still think it's ok as long as they are adult and the authoritarian feeling don't influence the decision to have sex.

I think you'd first have to define what moral can mean.

In the book of Lies and False Promise (the bible) incest is quite frequent. The story that comes to mind is the story of Lot and his daughters. After Lot's wife is turned a pillar of salt the daughters get Lot drunk so that they could "mate" with their father. Of course there's the story of Adam and Eve. There is no explanation on how the world was populated without incest.

Gross.

Non religious morality doesn't come into it. So long as its purely consensual then noone is harmed, so long as protection is used to ensure no procreation, you aren't exposing possible children to debilitating birth defects. Its still icky though.

Would not morality have to involve a degree of harm? Consenting adults who will not risk reproduction from the union/ activity would not be immoral. Morality stems from a social doctrine designed to control & prevent a perceived or actual harm. Thusly an adult, consensual, no
risk to pregnancy coupling would not be immoral.

According to the religious right, incest is perfectly normal in a close Christian family.