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SAB: Genesis
Dwindling in Unbelief
-God's Killings in Genesis

Genesis 4:17
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.

"And Cain knew his wife." That's nice, but where the hell did she come from? The Bible doesn't mention any of Cain's sisters. Well, maybe he married his mom, or maybe God pulled another creation over in the next county. In any case, Cain and the mysterious Mrs. Cain have a son (another blue cigar!). His name is Enoch and he builds a city (population 3).

Although the Bible doesn't say, Joseph Smith provided the answer in "The Inspired Version" of the Bible, which was his very own "translation". Here's what it says:

And it came to pass, that Cain took one of his brother's daughters to wife, and they loved Satan more than God. Gen.5:13

So according to Joseph Smith (and he ought to know!) Cain married one of his nieces. He doesn't say, though, which of Cain's brothers was Mrs. Cain's father.

Another answer is provided by the Pre-Adamite theory.