Not much, actually. Sometimes the Bible makes very clear and specific statements regarding an issue that leaves little doubt. Other times, to arrive at the truth, we need to do a comparative study and come to a reasonable conclusion based on the evidence.

In the case of who Cain married, there is abundant evidence that he married his sister.

Eve was so named “because she was the mother of all living” (Genesis 3:20). The Bible says that Adam and Eve had sons and daughters (Genesis 5:4) other than the three sons that are specifically mentioned. It was out of these daughters that Cain must have found his wife. It makes sense, because there was no one else around.

The earliest inhabitants of the earth had no other choice than to marry their brothers or sisters in order to fulfill the divine command, “Be fruitful and multiply” (Genesis. 1:28). We know that before the time of Moses, it was not forbidden to marry a sister. For instance, Abraham married Sarah, his half-sister. Genetic vitality at the time was pure and strong, so there was no danger for intermarrying that would produce genetically endangered offspring. As the population increased and DNA began to corrupt further as a result of the curse, such marriages were prohibited. (See Leviticus 18:6-17.)