According to (Genesis 4:16), after Cain murdered his brother Abel, he went away from the presence of the Lord, and dwelt in the land of Nod, east of Eden. (Genesis 4:17) states that Cain got to know his wife, and she conceived and bore Enoch. But wait a second. Come to think of it how did Cain find a wife already? Where did this woman come from?
One important thing we should remember is that in the book of Genesis, humans lived much longer than they do now. According to (Genesis 5:5) Adam lived for nine hundred and thirty years; before he died. In accordance with the logic of the book of Genesis, Adam and Eve had to conceive children in order to “be fruitful and multiply,” as the Bible indicated, otherwise the human race would simply not exist. In that case, according to some Jewish traditions, Adam had 33 sons and 23 daughters.
The Bible does not in any way tell us how Cain found a wife and so it is non-known if he married a sister, or niece or even a grand-niece. None of those details were found in the Bible. But what comes from the narrative of the book is that since Adam and Eve were the first parents of all humanity, then Cain may have married a relative.
Not minding which generation Cain found a wife, someone at some point had to get married to his own sister.
Genetically, many biblical scholars agree that Adam and Eve were without defect (asides from original sin), and so the marriage between the two successive generations would not have produced genetically defective children. It was part of God’s initial plan in order to populate the earth.
Also, the Law of Moses was not in effect at this point in salvation history, and so sexual relationship between close relations was not only not forbidden but moreover necessary for God’s plan to unfold. This may have also been what happened with Noah’s family after the flood.
Apparently, some other biblical scholars believe there were other humans that existed besides Adam and Eve and that when Cain left Eden, he found other humans not related to him. There is a theory known as polygenism which argues that there were different groups of humans that existed independently of each other, and not from an original couple.
The Catholic faithful cannot welcome that opinion which maintains that either after Adam, there were true men that existed on this earth but they did not take their origin through natural generation from Adam. But it should be taken into account that more recent Church documents who have not commented on the issue, have left the door open to other possibilities based on recent developments in science.
The book of Genesis is understood to be symbolic in nature and not scientific, there are varieties of beliefs that can be accepted that do not contradict the spiritual meaning of the text.
Lastly, as it happens with plenty of this kind of stories, we don’t really know who exactly Cain married. The Church believes that the Bible contains everything we need for salvation, so the inspired authors necessarily omitted details for reasons that have to do with the last intention the text was written for.