So here we see that although Adam and Eve and their two son where supposed to be the first people, there were obviously other people who were not born in God’s paradise who could kill Cain or marry him.
Though I'm not a young earther the biblical answer for this is that Adam and Eve had 800 years or so of life. They had a LOT of kids...but just three are listed. All of these kids married each other and because they had a purer genetic line there was no problem with the genetics of it..
Where in the Bible does it say that Adam and Eve had 800 year of life? Where in the Bible does it say they had many kids? What the Bible DOES say is that their children produced 2 lines of descent. One is the Cain line which was in ongoing rebellion against the will of God, and the other was the Seth line that lived in obedience to God. And the Bible for sure says NOTHING about lack of incest genetic problems because of the purity of their genetics. It is so easy to say that the Bible could never be wrong in the light of more recent knowledge. It is so easy to come up with Biblical answers that are NOT in the Bible to “prove” anything you want to prove. There may be people in science with egos that stick to wrong answers, but plenty of others in science are there to point out their mistakes and generally be believed with enough evidence.