What was Cain's relation to his wife?

Cain’s wife is alluded to in Genesis 4:17, but where did she come from? After all, only three children of  Adam and Eve (Cain, Abel, and Seth—all sons) are explicitly mentioned in Scripture.

Leaving aside for now the question of literary criticism and whether the sacred author intended to give us an exact chronology of Adam and Eve’s descendants, we can say from the text itself that the Bible does not list all of Adam and Eve’s children. And specifically, there is a clear implication in Genesis 3:20 that all human beings are descended from Eve, just as all Christians honor Mary as their spiritual mother and thus the “New Eve.”

The typical answer that is given, then, is that Cain would have married one of his sisters. That is also the interpretation that has typically been held by rabbis through the centuries even before the time of Christ.

That Cain and Abel married their sisters is logically inferred from the Church’s doctrine of original sin. Original sin is passed on to all human beings from our first parents, Adam and Eve. As St. Paul says, “Sin came into the world through one man” (Rom. 5:12; cf. Rom. 5:19-20 and Catechism of the Catholic Church 402-06).

Certainly there is the issue of incest, which Leviticus 20:17 calls a “shameful thing.” The prevailing view is that it was considered a necessity within the first generation of offspring for Adam and Eve. St. Augustine provides a clear summary of how the Church has understood it:

“As, therefore, the human race, subsequently to the first marriage of the man who was made of dust, and his wife who was made out of his side, required the union of males and females in order that it might multiply, and as there were no human beings except those who had been born of these two, men took their sisters for wives,—an act which was as certainly dictated by necessity in these ancient days as afterwards it was condemned by the prohibitions of religion . . . and though it was quite allowable in the earliest ages of the human race to marry one’s sister, it is now abhorred as a thing which no circumstances could justify” (The City of God XV.16).

Courtesy of Leon Suprenant