Brother /sister marriages were common enough in some "royal" cultures, (for ex. ancient Egypt). Apparently their God's didn't have a problem with it. Although recessive genetics can be a b****.

Equine, canine and poultry breeders call it line breeding and it has caused genetic disorders. Ever bragged on your purebred puppy without questioning how it was produced?
First cousins marrying, for example Queen Victoria's progeny promoted hemophilia in her line. I believe whether something is moral or immoral has more to do with what particular society and time you are in.
We invented religion to help enforce rules against conduct that was seen as negative. We invented the gods for our own purposes. Back before we had science it could actually have some positive benefits to society. Or not.

There is a cultural taboo against incest not a religious taboo although in the bible belt of the USA marriage between first cousins seems to be common. The set up of most social animals makes incest uncommon. eg male hyenas will move to another group to mate. Some bird species eg swans mate for life so incest is unlikely.

It may be that we have several questions that apply and are being confused.

1. Is it a good idea for the individuals?, for example, when (if?) they move on to new partners they “divorce” . Will they have problems adjusting? They will have little if the support available to married couples.
2. Is it legal? Cultures wind up on many sides of this one.
3. Is it a good idea for society? Will it lead to clans, as happened in rural WV for instance?
4. Is it a good thing for the family? I would put in the slippery slope (gateway to father-daughter sex) in this item.
5. If they do have children who continue the “tradition” genetic defects will pop up.

Which of these apply to a definition of “moral”? And there may be many others.

Sex is never safe, when brounderies are not respected! Hearing stories from my peers as an older person...it does not bode well in old age! It appears to eat away at their mental health!

Ok, so we're assuming consensual sex, obviously. Between adults. Like, all parties involved are adults, not most of them.

Even if the potential for reproduction is involved, the potential for irreperable damage to the species is negligable, right? With such a diverse gene pool, regardless of how screwed up some subsection of the population gets through inbreeding (see Blue People living in West Virginia mountains...), there is a larger genetic base to mitigate whatever irregularities arise from the inbreeding of various small groups. As long as everybody isn't doing it, right? Although perhaps that's where the whole biblical provision came from in the first place. Too many villages full of people with hip-displasia and the cognitive inability to do much of anything other than fuck their siblings.

I mean, they inbred the hell out of dogs. You like your dog, right!? It might be a fantastic idea to selectively inbreed people. Just remove the ones with non-desreable traits from the gene pool. Although I'm pretty sure any sort of selective breed in humans is considered Eugenics, and that's generally frowned upon these days... Nazis fucked it up for everybody.

So, if you remove procreation from the picture completely (as the OP did)... then sure, why not? What about this consideration: what will children who grow up in an environment where Mommy and Daddy are Brother & Sister going to develop as their paradigm for romantic relationships? Since its obviously acceptable to fuck your brother or sister in this environment ("Mom and Dad are such hypocrites! Grandma says they've been hooking up since they were OUR age, but they won't let us do it yet..." ) what's to stop the kids from banging?

Assuming they are of the same age & maturity level, is this any more immoral than the parents banging? They would probably think its normal... enough... Keep in mind that aforementioned kids are probably not genetically related (unless adopted from same parents) as they cannot have come from original Brother & Sister, since we have already ruled out the potential for procreation in this example.

Synopsis: A moral acceptance of incest would lead to incest to become socially acceptable and potentially integrated into mainstream culture... but as long as there is no potential for children to arise from the union, it is no more or less morally acceptable than banging someone who isn't related to you.

In other words, if we ever find out the world is going to end in less time than it takes to gestate full-term, it becomes perfectly acceptable to fuck your sister.

P.S.- I am an only child.

Medically speaking I first ask; How actually safe is "safe sex" considering that almost EVERY form of contraception is, at the very best, only about 80-90% effective, except, of course, for total abstinence or surgical techniques such as; 1) a irreversible Vasectomy for males and a total hysterectomy for females, or, 2) castration of males.
Genetically speaking, incest is a very hazardous activity in any human relationship between brother and sister, father and daughter, mother and son or first cousin to first cousin with a direct genetic link.
That is why, in the wild , most mammals and many other species will drive out their young once they are old enough to survive on their own to limit the chances of the genetic pool becoming tainted by incestuous in-breeding thus causing deformities ranging from very minor ones right through to the most horrific.
DNA strands, which every living thing are made from, are very delicate and very easily 'damaged' ergo Incest is NOT a religious Morality Issue but an Ethical and Genetic Issue for a living things and, on a personal point of opinion here, I think that with all the advancements in Invitro Fertilisation and random donations of sperm, etc, human kind is slow but inexorably heading down the road to a destination where in-breeding and the genetic disasters will become the norm unfortunately.

you forgot "neither". not every action in the world is one or the other, some are not a question of morality.

is it moral or immoral to tie your left shoe first?

I do not view it as an issue of morality. As at least one comment pointed out, it's an evolved trait that makes us feel that way about it. it is not an issue of "morality", it's an issue of internal mechanisms that make us feel squeamish about it. and the reason that evolved should be plain to see in any heavily inbred community.

To me it's immoral, because all sex must include complete consent, and I don't think people who are related can give true consent, because of power dynamics, etc.

Don't. Fricken Hanoverians should be enough of an example. Safe sex? No, you mean safer sex. It's similar to the term "less than lethal" in law enforcement.

I think it is immoral if it is parents and children, but not between two siblings if they are adult and consenting. I wouldn't do it, but I think it is okay between brother and sister and it started in adulthood and both are of similar age. When power dynamics are involved, it gets weird and wrong. So there have to be caveats.

In and of itself, incest is not per se immoral, although a controversial area and as mentioned there are very valid reasons why we have instinctive and social taboos against that in most circumstances.

A good treatment of some these issues turns up in the novels of Robert Heinlein, such as "Stranger in a Strange Land" and "Time Enough for Love".

What is always a problem in any relationship, especially a sexual one, is any large power imbalance and any lack of consent or incapacity of one party to validly give consent. Then it's abusive, and in many cases incestual relationships would also cross that line; but that is where the immorality stems from, and it is exactly the same issue as for example a teacher forming a sexual relationship with a current student of theirs, even if above the local age of consent.

One of the things that seems to be dramatically changing in recent decades is how we approach relationships and how we think about morality around that. A huge change in living memory is the development of reliable contraception, which I believe has demonstrably resulted in a massive drop in infanticide and child abandonment compared to past centuries, and also (for the most part) effective and safe treatments for most STDs which again is a game-changer.

The other perhaps less visible change that I believe is equally seismic in the social changes happening is ever-increasing typical lifespans for humans, which has massive implications for our patterns of relationships.

A surprisingly thought-provoking question. Not least because it provokes a emotional response ("no, no that's disturbingly wrong" ) before allowing oneself a rational response "why is it wrong? Because it is immoral?" When morality is a human invention. Interesting that siblings are mentioned. Incest can also involve father and daughter/mother and son. Different reaction entirely. Even if adult and consensual surely an explotative relationship on behalf of the parent. Yet often younger women seek out a father figure and as often younger men a mother figure as a sexual partner. Latent incestuous desires or no? Egyptian pharoahs regulary married their sisters. Greek myths punished incest between parent and child (usually because it involved patricide to get to the mother). The Doors , The End ... Jim Morrison sings " Father i want to kill you, Mother i want to ... (screams). Culturally taboo to protect the species seems to be the way to go, with regard to siblings at least. (I am discounting child abuse in all of this and only talking of consensual adults). I believe in compassionate understanding rather than retribution and punishment in such cases. As i said, thought-provoking ...

I can't vote because the question isn't based in moraity or lack of it. The answer lies in the history of man as we know it.

I can admit that I don't have enough of an education to be able to address this dilema.

Slightly off topic:

Anyone know how the Lanisters make a king sized bed?

They push together two twins!

I have scanned the comments and present this scenario: You have met and married someone in your town or city and you've had a wonderful marriage and raised several beautiful children and you decide to go to one of those DNA site to discover your family history and find out that the person that you've been with is actually a sibling. Now what? They believed that they were morally right. The unspoken truth in society is that many people's first sexual experiences have been with family members. We shouldn't have a problem with it if it's done respectfully. If it leaves a bad taste in your mouth then go wash your mouth with atheist brand mouthwash that's made with plenty of ration. lol. Does morality equal respect?

it's just a cultural issue. just like not boning 15yo's is a cultural thing.

I don't see anything inherently harmful about the act as long as both are of about the same age. The repugnence is totally a cultural aberration brought about by moralistic religious teachings.

There's a thing called GSA - genetic sexual attraction. Now and then there is a news article about a child given up for adoption that when grown seeks out its birth parent and falls in love. Apparently that attraction is pretty strong when you read the stories and listen to how the participants talk about one another. Granted the circumstances are vastly different than a brother and sister being raised together, but in the eyes of society they are equally taboo.

[en.wikipedia.org]

Christians are hardly in a position to argue the morality of incest, seeing as the bible is rife with it. I believe it is not my remit to condemn 'victimless crimes' but feel in general, the practice of incest is potentially devolutionary and therefore not to be encouraged. It is also difficult to argue a moral standpoint on such issues without addressing the concept of morality itself and that is an inconstant as old as mankind; far older than any Christian ideal.

Why not? Not your business

If it's mutual and there is no manipulation involved, I can't find any immoral aspect in it. But one line of argument can be that the relationship between a brother and sister is destroyed through this choice and consequently the action. It's more likely should be discouraged unless they have fallen in love and are planning to continue a conjugal relationship.. which again will be criticized by the society and culture.

It remains immoral for other reasons. One argument falls into a simple evolutionary argument. The higher risk of serious defects in a child make the act dubious in my eyes.

Also, regardless of from where you derive tour morality, societal ethics still fall heavily against it. This argument is pretty flimsy, but social taboos exist.

That having been said, out of sight out of mind I suppose. If I don't know aboit it and they aren't hurting anyone I guess it's no different from any other kink.

Still though,

Why would you want to traumatize and ruin a persons life.

It was found long ago that inter breeding was dangerous. So incest was banned by the rulers and agreed to by the clergy (I think there is something in the bible about it). The problem is that they did not understand the genetics of the situation. So we now have the situation of where two adopted girl and boy cannot get married because they are siblings.

Leviticus 18:24 says, “'Do not defile yourselves in any of these ways, because this is how the nations that I am going to drive out before you became defiled.” What were “any of these ways”? Chapter 18 focuses on immoral sexual practices, including incest, bestiality, same-sex activity, and adultery. In addition to prohibiting sexual immorality, Leviticus 18 addresses the heinous practice of sacrificing children to Molech verse 21