Not going by any religious beliefs it does not seem natural!

I voted “immoral.” In the rare instance of adult, consenting adult siblings engaging in sex, I don’t have an issue and it is not necessarily immoral. This scenario is more likely and why I voted “immoral.” If either or both cannot consent (age, capacity) or an unbalanced power dynamic, then it is immoral. But I’m left with why in the heck would siblings want to engage in sex?

Cain and Abel can best answer the moral aspect, other than that, to my way of thinking, adult, consenting sex should be shared freely among people...if you are going to believe in a god, then accept it as a gift...

It's irresponsible on the genetic level, because knowingly creating a life that is very likely to have all kinds of recessive defects expressed is not a good thing. Unless you can be tested and have all the genetic drawbacks ruled out.

On an emotional level, it's a breakdown of the normal dynamics of the brother-sister relationship. Usually there's a lot of unspoken or even only unconsciously known power dynamics (among others) in that relationship, and adding that intimate wrench to the gears is likely to cause some larger issues. If you break up with an unrelated BF/GF, they can leave and that's the end of the story. If the BF/GF relationship with a sibling goes wrong, family reunions will never be the same, and you can't easily just cut that person out of your life.

“Morals” are often simply social mores that have developed because they benefit the tribe. Incest was harmful to the tribe based on issues related to kinship and gene strength, So in that sense incest is culturally taboo, which is the linguistic equivalent of “immoral.” Limiting the question to “safe sex” does nothing to alter the morality because individual instances do not affect sweeping social perspectives. To test this, ask yourself whether you would mind reading about your activity in a certain domain in the newspaper. If you would, then that activity is culturally taboo or “immoral.”

Moral noun "a person's standards of behavior or beliefs concerning what is and is not acceptable for them to do."
So where does this standard come from?
Our society, for lots of reasons, family structure, inbreeding, and so forth.

Why do so many Atheists FAIL to see these are ALL strawmen arguments from theists?
They believe in a God and a religion with its laws, Morals and precepts.
We do not.
Since they cannot PROVE the God and hence the validity of the Laws and precepts, they instead construct endless strawmen. Evolution, Cosmology and all sorts of moral arguments.

"If there is no God why is safe sex between brother and sister immoral to an atheist?"
No answer to this Proves God, it is a strawman.

In the long run, it's not something society wants to happen on a regular basis. It's deemed immoral for the purpose of stifling the practice. I voted it as moral, only because the dangers are overblown. I never found my sister in the least bit attractive, so the thought never crossed my mind.

I would say that the act itself is not really moral or immoral in and of itself, however, the production of offspring from such a union is not to be encouraged. The children would be more likely to have or spread genetic disease and would weaken the overall genetic diversity and strength of the human species.

There is an inhibition in humans and other mammals to have sex with one whom you have known since infancy. Doesn't have to be related.

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.

If there is no power imbalance or manipulation in the form of grooming, if it is consensual and between adults then I haven't got a problem, even knew a pair of sisters who regular satisfied each others needs rather than bring a strange man into a household with a small child, they just figured it was the safer alternative. However all the conditions have to be met and they were in that case, usually they aren't.
Genetically speaking line breeding is often used in studs to fix traits so if you had two particularly fine specimens with highly desirable traits I wouldn't even insist on safe sex, but you'd want a full genetic screening first which is kind of hypocritical considering society has no problems with two individuals with known inherited illnesses from having children.

More men will vote for moral than women. This question was probably brought to you by a male, which is why people are on here trying to hook up.

This has become more of an issue in modern times with the rights of adopted people to find their biological family. Apparently There is a 25% chance of sexual attraction in the case of each individual. Doing the maths that means that 6.25% of the times it will be mutual.

I consider it immoral, in the sense that it can be harmful for society. I remember learning in sociology class that incest is a taboo because family members usually have very easy access to one another, and also that if such behavior became normal, that it could cause confusion in social roles. this makes sense to me. However, I don't believe it is something gross or horrible; it's just something we should not do for good reasons, and it is taboo because if it weren't, it would probably become commonplace. But for those who consider truly terrible and disgusting, what is it about it precisely that makes it so? I don't approve of incest, but it doesn't horrify me either.

If incest is practiced in a manner in which it is not abuse then I (putting my gross factor aside) would say it is Amoral.

I think its not a moral issue at all its one of genetics you haven't such a wide gene pool between you so it just makes sense to have children with non-close relatives

I honestly am indifferent... The only real problem with it is the genetic defects that are bound to happen from incest otherwise the actual act of having sex with a relative isn't really any of my buisness

I don't judge it. I hold the view that what consenting adults do is nobody's business. If that can be true with gay relationships, that can also be true with incestuous relationships. I wouldn't advise conceiving children for genetic reasons but there are options. Birth control exists. There are also many cases where it's gay incest. Nobody has to understand it. But then again when does anybody understand what love is going to do?