The Bible tells us that Adam and Eve were the first man and woman and Cain was their first son. His wife must therefore have been his sister. This seems strange (or worse!) to us but we know that that is where the human race all started, and this incest was under God's control and guidance.
Genesis 5:4 tells us:
"After Seth was born, Adam lived 800 years and had other sons and daughters."
It reads as though there were several brother/sister couples. It wasn't quite the same following the Flood, as the Bible tells us that there were eight in total who were saved in the ark, or four couples. They consisted of Noah and his wife, and their three sons, who were therefore all related, but the wives of the sons were not. Therefore the sons children would be cousins and marriage between them was not incest.
At the time of God giving the law to Moses, the practise of incest, as well as some other close relationships, was stated to be wrong (refer Deuteronomy 27:22) because by this time there were sufficient people for it to be avoided. At this time too, God brought in the instructions regarding respecting parents and elders etc, so it was all part of establishing the family unit as the basis of society.