Adam & Eve many children, not just Able, Cain, and Seth. Adam lived to 930 so I'm sure they had quite a few kids. Yes, there was incest involved between the brothers and sisters... There had to be. Adam & Eve were God's most perfect creation. They were genetically flawless. They contained the genetic differences we see among the many "races" of people. They probably had children that looked white, black, asian, latino, etc.. So, mixing between the sibling would not cause genetic defects like we see today.