Who did Adam and Eve's son Cain marry?
According to Christian mythology, Adam and Eve were the first people created. So their children were living alone in the world.
In Genesis 4:16-17 it says that Cain left Eden and went towards the east to live. There he met his wife and got married and had a child. Where did that woman come from? Weren't Adam and Eve the first people created?
To address the claim that Cain moved away and then found his wife, this simply isn't stated in the text. Perhaps the confusion is on the phrasing. Genesis 4:16-17 doesn't say that Cain met his wife in the land of Nod. Here is the King James Version of those verses:
And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden. 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 phrase "knew his wife" is a construction used repeatedly in the King James translation. "knew" here doesn't mean "met", it means "had sex with". Later translations usually make this a bit more clear to the modern ear, using phrases like "made love to", instead.
As I understand it (and I'm not a scholar of Hebrew), לדעת has many meanings, including both having knowledge of, and having sexual congress with (possibly euphemistically). To me, the intended meaning seems clear here (not all people I meet conceive and bear my children, after all). And to the best of my knowledge there isn't any real debate of the correct interpretation of this passage.
More discussion of this usage can be found on English Language & Usage: Why did Old Testament scholars choose to employ “to know” in a sexual sense?
There is nothing that precludes Cain having already been married to one of his sisters at the time that he left for Nod, and that he brought her with him.
In the ancient greek version of Genesis from which the King James version was translated (if I am not mistaken about the origin), it states met. In past tense