After Cain murdered Abel and God cursed him for his sin, the Bible says…
“Then 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 bore Enoch” (Genesis 4:16-17a).
One question that always comes up in regard to the Bible is the matter of Cain’s wife. Who is she? Where did she come from? The answer is she was Cain’s sister.
Science has proven through the study of genetics that we (the entire human race) are indeed descended from one man and one woman. There is no question to that fact. Therefore, Cain married his sister.
Then the question comes up about children from incest and the genetic issues that can surface. Genetic abnormalities even within children from incest are rare. Though there may be a higher risk factor. The media/entertainment just hype it as being common. For the most part, our culture is repulsed by incest (the Lannisters don’t seem to care).
Speculating, human DNA at the beginning was “new” and probably did not have problems with mutations or degradation. The risk of abnormalities was probably much less than it is today after generations of genetic replication that is impacted by environment, chemicals, and exposure to various things like radiation. I’m sure there are probably other factors as well that can have an impact.