“And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.”
The subject of Cain’s wife is often used as an excuse for not believing the account in Genesis. The problem is stated thus. Cain was the son of Adam and Eve. Cain killed his brother, Abel, so who were the people East of Eden from which Cain took a wife? Did God make lots of other people in different parts of the world? Or is this just an inconsistency in the Bible – part of its “inherent weirdness”, as Richard Dawkins once put it.
It is in Genesis 3 that we read about the conception and birth of the first two named children of Adam and Eve, namely Cain and Abel. In Genesis 4, we read of the encounter between Cain and Abel, where Cain ends up murdering Abel. However, Cain and Abel both appear to be adults at this point in the account. Genesis 5:4 tells us that Adam and Eve had other sons and daughters. Cain, Abel, and, later, Seth are simply the only ones named.
So there could have been plenty of time before the events of Genesis 4 for there to be a reasonably sized population. And, therefore, it is logical to suppose that Cain’s wife was his sister.
Although the Bible today forbids such close marriage, this is because of the genetic damage it can cause. In the second generation, there were, as yet, very few genetic mutations, so there would have been little genetic danger in such a marriage.