We end up with far too many stupid people when they keep screwing their siblings....

That was the only way isolated tribes multiplied - immorality came later as developed.

Because it's just sick and wrong.

I was a psychiatric RN for over 30 years and abuse of women and children was my main area of expertise but I have to recuse myself in part. If not sn act of aggression, and consensual that can be different although I might have difficulty endorsing it. I would have gone to bed with my cousin in a minute although we haven’t been in touch. Never my sibs and by the way I’m not a Christian... however I might hop on to bed with you as you are very attractive and sexy

Morality is a subjective term dependent upon the predominate views of the society or group within which the action is being judged. In my group of one, it is moral. If my group were larger I might have to change my answer, but I would have to know the disposition of the group. I might add that it would need to be a unanimous answer amongst the group.

There's no morality about it. Its just fucked up. It has so many genetic and medical downsides that I would never condone it. But i think 2 consenting adults should be able to do what ever they want as long as it doesnt harm others

Ruins the sister-brother relationship, one that is based on traditional values excluding religious beliefs.

WTF??

It's Nature's Way of keeping us from having two headed 12 toed babies

Inbreeding magnifies genetic defects.

If children are produced it is bad for the genetic diversity of the offspring. Look at the Amish. They are descended from a very small gene pool and for the most part only marry other Amish, so they have a very high number of genetic diseases.

I define good as that which advances life and evil as that which impedes life. Incest impedes life and is therefore bad.

If it is safe sex, you're skirting the edge of morality and immorality, but on the right side. Barely.

Now, thinking that you have a right to decide that for others is highly immoral and should not be tolerated.

It was certainly the way the Ancient Egyptian royal families kept their bloodlines pure as they believed that they were descendants of the gods. See the work of Frank Ruhli et.al - American Journal of Physical Anthropology 157(3) · April 2015 

For too long now Governments, The Church and society in general have just been obsessed... absolutely obsessed with whom puts what part of their body in whom. If it's safe and mutually consensual - Butt the fuck out is my honest opinion.

Just like food... "The only people so obsessed with food are the anorexic and the morbidly obese, and so it is with the Church and sex" [nods to Stephen Fry]

Please tell me, where in the Bible it says it's a sin? I've never heard of such a Bible verse. People are so stupid when it comes to religion. Most practices of the Christian religion, is not even in the Bible. Most of it rationalizations of someone's "interpretation". The fact is, in the Old Testament there are a hand full of direct commandments, and most of them are ignored today.