Genesis 4:17 tells us, “Cain knew his wife, and she conceived and bore Enoch.” That is the first and last mention of the wife of Cain in the entire Bible. Naturally, this raises some questions: Who was she, and where did she come from? The Bible names only three offspring of Adam and Eve: Cain, Abel, and Seth (Gen. 4:1–2, 25). However, Genesis 5:4 says that Adam “had other sons and daughters.” Since all mankind came from one man (Acts 17:26), then Cain’s wife was either a sister or a woman from the line of Seth. It is possible that she might have been from the line of Abel. We are not told that Abel had children, but since Cain was concerned about being slain because of his killing of Abel (Gen. 4:14), some of those people might have been offspring of Abel, seeking to avenge their father’s death.
Gaps or omissions like this in the biblical record seem to encourage speculation among readers. From the Jewish sources (Pirkei of Rabbi Eliezer 21:7), we read of speculation that Abel had a twin sister named Awan (the book of Jubilees 4:11) who was very beautiful. Because Cain desired to take her from Abel and make her his wife, he killed Abel. None of this speculation, as interesting as it might be, should be taken seriously. All we can know from Scripture is that Cain’s wife was his sister or another close relative.
The idea that Cain’s wife was also his sister raises the question about incest. Though the idea is odious to us, it would have been necessary in the first generations. As the population of the earth increased, however, it would not have been necessary and was eventually prohibited in the law of Moses (see Lev. 18; 20).
Many commentaries on Genesis do not address these questions, in part because they are considered to be outside the scope of what the commentary is intended to do. More liberal commentators may ignore these questions because they consider the opening chapters of Genesis to be mythological in character and thus not to be taken seriously in any case. The comments by C.F. Keil in the Commentary on the Old Testament helpfully sum it up: