Do they give a word for sibling sex?

The word is Incest.

Sibling sex is incest.

The ancient Egyptians called it "having a good time." All other societies found it to be very disturbing and was considered taboo. Incest remains the most common taboo on the planet. If you have a desire to try incest be sure to dial 911 for up to date information.

... on sibling sex ...
And the reason for that negative attitude towards incest is the damage it may have on the DNA of possible offspring by amplifying the influence of faulty genes.
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Even in this day and age and with all these means of contraception the resistance against incest remain intact. That has more to do with protecting members within a family against unwanted sexual abuse, special the smallest and the weakest.
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It also shows you how thin the layer of social and moral veneer is of some people. So thin that they misuse just that group of small and weak victims.
;)

Incest and it's wrong

incest and it's wrong
You are right on that it is wrong. But you should always explain WHY that is wrong!
:)

You are right on that it is wrong. But you should always explain WHY that is wrong!
:)There is more than one reason why its wrong. And to the average person its obvious. Beyond the genetic deficients that it would produce (due to recessive genes)... its also morally and ethicly wrong.

Sibling sex... is a no no.


That is very very wrong
Its called incest.

Keep it out of the family

It's interesting to me that incest is such a moral taboo--yet it happens in the Bible, at least once that I can think of off the top of my head (Lot and his daughters).

Royalty for centuries married siblings and cousins, to keep the bloodlines pure.

So... it's morally and ethically taboo in OUR CURRENT SOCIETY. Outside of Western culture, and outside the times that we live in, incest was accepted in some cultures.

This is not to say that I'm promoting it---just saying that we can't necessarily say that it's completely wrong, without knowing the circumstances behind it.

Either way, the OP's question has been answered. I move that we take a discussion of whether incest is evil to a discussion elsewhere.