Where did Cain get his wife?

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One of the most commonly asked Bible questions is: Where did Cain get his wife?
Yet, the answer is quite simple;
He married one of his sisters (see Genesis 5:4).
Understand that the Bible only lists the births of principal characters, such as Cain, Abel, and Seth (from whom we descended).
But Adam and Eve obviously had many, many children, both sons and daughters.

Does marrying a sister sound immoral or illogical?
Well, it was a common practice in Bible times.
Take for example the righteous man Abram (AbraHam). He married his half-sister Sarah, for their common father was Terah (see Genesis 20:12). Thereafter, AbraHam sent his servant to his closest relatives to take a wife for his son IsaAc (IsaAc’s first cousin, Rebecca… who was also a granddaughter of Terah). This pattern of taking close relatives as wives was then followed by both of IsaAc’s sons, Jacob and Esau, who also married first cousins that were descended from Terah.

So what we call incest today wasn’t illegal or immoral in early Bible times.
For more information, see the subheading, ‘Incest’ in the linked document, Christian Morality.