Did Adam and Eve's children practice incest?
The Genesis account of Creation provides mankind with just the bare facts we need to know about our origin. Further facts are revealed progressively throughout Scripture. The Creation account then concludes with, "Then God saw everything that He had made and indeed it was very good" (Genesis 1:31). Since Eve was made from one of Adam's ribs [Genesis 2:21-22], she would have been a clone of Adam and, had there been any genetic mutation in Adam, this would have been reproduced in Eve and expressed in their offspring. However, we may reasonably conclude that there were no mutations, and the very first commandment given to them was "to be fruitful and multiply" [Genesis 1:28]. However, the business at the tree of the knowledge of good and evil took place long before there were any children.
The account then continues where God confronted the guilty pair at the tree, but they did not confess their guilt or plead for forgiveness [Genesis 3:1-13]. God then cursed the serpent, imposed reproductive difficulties upon Eve and "cursed the ground for [Adam's] sake" [Genesis 3:17]. From that moment, everything that Adam – and mankind since – ate had grown in the cursed ground. Cell by living cell, Adam began to very slowly change from his initial state of eternal perfection to mortal imperfection, and he finally died at the age of 930 years [Genesis 5:5]. Nevertheless, Adam and Eve's immediate offspring would have been very close to physical perfection while brother-sister marriages were the only unions possible! Further, according to the genealogies given in Scripture, pre-flood longevity was about the same as that for Adam, so families were very large compared to those of today. Brother-sister unions were not only unavoidable, but they undoubtedly became traditional and expected.
Following the Genesis Flood, other sources of genetic defects such as harmful radiation were also imposed upon mankind. Finally, almost nine-hundred years after the Flood, God gave Moses a list of near-relatives, including brother and sister, who were forbidden to marry [Leviticus chapter 18]. The leaders of many ancient nations had always married their eldest sister as first wife to ensure the kingdom stayed in the family. For example, Pharaoh Akhnaton [reigned 1379-1362 BC] had been the product of such a union and from his preserved image he is recognized as having been genetically deformed. Over three thousand years later Charles Darwin married his first cousin and had ten children, three of whom suffered very early deaths – strongly suggestive of expressed defective genes. First-cousin marriages have long since been declared illegal in England.
Today, we all carry those mutated genes, and when two people marry who, by chance have the same defective gene, that gene will be expressed in their offspring. The chances are increased enormously when the married couple are closely related, especially when brother and sister.