Good question. Here is an answer by the lovely folks at GotQuestions. I think it answers your question directly. They write:
The Bible does not specifically say who Cain’s wife was. The only possible answer is that Cain's wife was his sister or niece or great-niece, etc....(*read the rest of the answer in the article)
Since Adam and Eve were the first (and only) human beings, their children would have no other choice than to intermarry. God did not forbid inter-family marriage until much later when there were enough people to make intermarriage unnecessary (Leviticus 18:6-18). The reason that incest today often results in genetic abnormalities is that when two people of similar genetics (i.e., a brother and sister) have children together, there is a high risk of their recessive characteristics becoming dominant. When people from different families have children, it is highly unlikely that both parents will carry the same recessive traits. The human genetic code has become increasingly “polluted” over the centuries as genetic defects are multiplied, amplified, and passed down from generation to generation. Adam and Eve did not have any genetic defects, and that enabled them and the first few generations of their descendants to have a far greater quality of health than we do now. Adam and Eve’s children had few, if any, genetic defects. As a result, it was safe for them to intermarry.
[Answered by Pastor HM]
