Incest, (from the Latin. in, not, and castus, chaste).
You can mess about with the margins of incest, whether you can marry first cousins and all that. But at the core of the nuclear family, the incest taboo is common among all societies, modern or primitive. It is the one anthropological universal rule; societies can differ in every other respect, but you cannot commit incest with the close members of your own family.
To quote Levi-Strauss, "the prohibition of incest can be found at the dawn of culture... [It] is culture itself."
Marriage Incest
Except, or should I say EXCEPT! A deliberate confusion has been allowed to
stand, for incest can refer to both the rules for marriage, and for the rules
for sex between family members who are not spouses. And while it may be true
that incestual marriage is always prohibited (though as usual, Royal Families
in many societies have flagrantly broken these rules), incestual sex is a
lot lot lot more common, and the edges are much more blurred.
Sexual Incest
It has been estimated that in the US, 20% of girls and 7½% of boys
have incest experiences. With uncles the most likely to be involved, followed by father,
brother and stepfathers.
See the universality of incest, though it argues that the corrected figure for incest should be 30% and 11% — this assumes overall abuse experiences being molested (by anyone) of 60% for girls and 45% for boys, of which half were incestuous for girls and a quarter incestuous for boys. This does seem very high, and begs the question — what is abuse?
Now, however common sexual incest might be at the moment, and with all the pedo panics around you'd have to suspect it's getting less so just now, in the past … well, just consider:
× Communal sleeping in the same room
× Communal sleeping in the same bed
Modern attempts to recreate communal sleeping have certainly suffered from accusations of gang rape and incest, as in the Israeli Kibbutz movement. And if it can occur in as high a minded movement as that, with its intense religious or Socialist principles, then it can assuredly happen anywhere.
For a different perspective, The Catholic Church says: Incest is sexual intercourse between those who are related by blood or marriage. Its specific malice is contracted by such unlawful commerce between those related within the fourth degree of consanguinity or affinity, as computed by canonists. The guilt is incurred not only by those sinful acts which are, as theologians say, fully consummated, but also by incomplete acts.
So I hope that's clear.