I couldn't vote because the question is particularly skewed and unclear.
First I would need a working definition of morality - are we talking about wellbieng or something else?
Are the siblings thinking adults of an equal standing who can make an informed decision of consent?
And why is the question specific to brother and sister? What about brother and brother or sister and sister? Or even brother, sister, brother, brother and sister for that matter.
Several posters have highlighted concerns about the possibility of birth defects in the event of unplanned pregnancy, but I feel this is just a distracter from the initial question and allows an emotional response and not one based purely on logic.
If you take the view of some religions that sex is purely for procreation then it is probably unwise to breed with your siblings.
If you think that sex between fully consenting adults just for pleasure is acceptable, then I see no reason why sex between siblings should be considered 'immoral'. Our socialisation and the norms of the societies in which we live will impact on our yuk factor and make it difficult for us to think rationally on such emotive issues. Probably not much different than asking a Christian why they believe in God.

If for any other reason, aside from the birth defect/health issue, I just find it disgusting and unnatural. I can't imagine this occuring between people who weren't at least pretty sick in the head.

JUST WRONG! Why do you think their kids have birth defects!

havent studies shown that it always leaves people worse off even if no kids are produced?

Because my Family Tree should Branch!!!!

CONSENSUAL, SAFE sex is nobody's business but the the participants!

NO. This argument leads to one thing. Making it ok to abuse children by grooming them for years to be sex partners. Its grotesque.

It is an Ethics issue. That is just not good for Civilized people to be doing.

I find this gross because i was raised to believe it is gross. People are taught from birth what is moral or immoral . There is no answer to this question because morals are derived from cultural and society upbringing. Incest occurs in the animal kingdom

Neither. Incest takes place in many societies and over 400 animal species. Is first cousin sex/married incest? Hundreds of societies do it. Best to avoid it in the immediate family for emotional reasons.

There is incest in most species.

How is this a question? It happened in the Bible it was condemned. Maybenif last two humans on earth and you need to repopulate but even that’s fucked. I’d clone first. Sick question...

No point in safe sex with a relative. Stupid question. Only reason to have sex is breeding. Unless every other woman on earth is dead there’s no moral or any reason to breed with a relative. Only to repopulate. Seriously, this is a disturbing question. I argue, should I let you all die since you’re not smart enough. To figure this out without help. Maybe we should let sea life like octopus’s take over, humans in general seem quite stupid to me. Wtf you even as this for you know the answer. My god the world is dumb and blind.

I heard it said all the world leaders are related to royalty , all the presidents having the same bloodline . So if that is the case wouldnt they be interbreeding and having incest relationships to keep the bloodline pure?

Where is the vote for "Neither moral or immoral"?

Morality is a human construct and not readily measured, and context has a lot to do with many "moral" decisions. Children from incest have a larger chance for genetic issues, so it probably isn't the smartest choice for a child bearing partner, but if you aren't going to have children, who cares? It is not my business to tally who is fucking who in this world, and I wish everyone else would stop because it shouldn't matter to anyone but the small minded who wish to gossip.

I have to say my brother sister and I were abused sexually by our much older uncle; so knowing that that is the most common occurrence of incest it is IMMORAL.

Having said that our DNA and genetic make up passes along our genes, and humans are susceptible to damaging genetic combinations which cannot be weeded out by stronger genetics when there is incest resulting in pregnancy. Wild animals display almost no signs of incestuous relations ships, unlike homosexuality.

I voted immoral because of the genetic concerns in future generations. I qualify this however by saying that safe sex practices and a "healthy psychological relationship" (if that's possible) would have to be used. This is something that I feel isn't practiced in most incestual relationships therefore incest in the practical sense is immoral.

Two non-religious issues with incest, as I see it:

1. The potential for genetic problems in any offspring produced.
2. The possibility of parents or older siblings grooming children for sexual activity. Consider a father raising his daughter to be his sexual partner the day she's legally old enough to consent, and this being perfectly acceptable in law.

The former, I will leave to science. The latter is a violation of the kind of trust that a child should be able to place in close family.

I find it rather tragic when two people meet, become romantically involved, and then discover that they're related closely enough to make their relationship incestuous. There has been no grooming at that point. Just a twist of fate that made their relationship unacceptable.

What's particularly interesting is where the boundaries lie. In the UK, for example, sex with a first cousin is perfectly legal. Elsewhere in the world, it's considered incestuous, and carries the same legal consequences and moral outrage as a parent and child or a pair of siblings.

it is moral because it is between two consenting adults. otherwise you could ask the same question in the context of same-gender sex. it all depends on the dogma and societal do's and don'ts of your culture... some say it's immoral for two men or women to have sex. awkward, but not inherently right or wrong IMO...

Perhaps mine is not going to be a popular opinion, but my moral compass is governed by the principle of doing no harm. If incest is consensual and they aren’t trying to reproduce, then it does not harm anyone and both consenting adults have a right to be happy.

You should ask said theist how Adam and Eve populated the earth. Or remind them of the daughters of Lot.

There are two types of incest, consensual and forced. Forced could also mean that the more vulnerable participant was manipulated verbally and/or emotionally. Consensual incest implies that both parties are fully aware of the consequences of having a child with genetic aberrations is increased. As long as each party feels mutually respected and has the option to stop at any time, their incest is none of my business.

If you were in an adult consensual sexual relationship with your sibling, would you be able to tell your best friend? Your parents? Talk about it with the girls over cocktails? Laugh about the queef you let loose after getting pounded all night by your BROTHER?! Man, I don't know about morals but I do know if you hide or lie about things, it's usually because you know it's not right.
I don't have brothers but have thought "if I had a brother and he was a hottie, would I ever find myself sexualizing him?" I can be a bit of a floozy! But I'm pretty sure I wouldn't be able to introduce him as my new boyfriend at the office party. "Oh, tell us all how you met!"
"Mom brought him home from the hospital when I was 2."

I said immoral and it has nothing to do with religion. On the rare case that 2 adults not raised together meet and have sex, then find out they are siblings, either half or full, it's just some shitty luck. If they want to continue that relationship, then by all means knock yourself out. If they are raised together as brother and sister, then that's just gross and you need your head examined. Having sexual feelings for a sibling should not happen. It's part of being human to contain different and deeper emotions towards parents, grandparents, siblings and our own children. We love them, protect them and respect them. Animals in nature do not have this. No sexual feelings should come into play unless you are mentally unstable.

I would ask him, "Was it moral or immoral when Josh Duggar... Christian... molested his sisters and Mike Huckabee... former pastor... defended him?" There was no penetration... that's about as safe as it gets.

Are you nuts?