We read in Genesis Chapter 4 about Cain murdering his brother Abel, and then Cain left the area with his wife and went to the land of Nod. Where did Cain get his wife? The answer is simple: he, and all of his siblings, married their female relatives. Now, many people may not like this idea, and even RTG could not fathom marrying any close relative, but the fact of the matter is that there were no other people on the Earth at this time in history except for those directly descended from Adam and Eve. At this time in history, everyone was under the curse, but the harmful effects on the DNA had not spread to a dangerous level, so marrying within the family would not have produced genetic disorders and other problems like it would today. When the population on Earth had grown to a large number, God put a stop to inter-marrying (Leviticus 18:6), although technically we still marry our relatives.
Adam and Eve had two sons named Cain and Abel, but they also had other sons and daughters (Gen. 5:4). At this time in history, everyone was under the curse and their bodies were affected by sin. The rate of harmful mutations in the DNA had not spread to the extent that it is today. Therefore, it was safer at that time in history for people to marry within the family. They had no other choice because there were no other people on Earth except blood relatives. So in short, Cain married his sister, or possibly a niece. There was no ethical, moral, or biological problem with this. This practice of marrying within families continued for 2,500 years until Leviticus was written and God’s law forbade marrying within the family (Lev. 18:6). By this point in history, there were plenty of people on Earth to allow a selection of spouses outside of the family of origin, and most likely the harmful mutations were more prevalent thus marrying a close relative would result in having a deformed or otherwise unhealthy child. That is the reason we do not marry our close relatives today—we are at high risk of passing on harmful, recessive traits to the child. Some people do not like the idea of God allowing marriage within the family, but consider this: Although today we don’t marry our sister, brother, or even a close relative, we still marry a relative because we are all descendants of Adam and Eve.