There is one very important part of the scripture that Young Earth Creationists seem to gloss over. Gen: 4-15,16,17. ...”Then the Lord put a mark on Cain so that no one who found him would kill him.” “So Cain went out from the Lord’s presence and lived in the land of Nod, east of Eden.” “Cain lay with his wife, and she became pregnant and gave birth to Enoch.”
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. Bishop Usser’s calculation was based on the “begats’ listed in the Bible, but we have no information on how long these other humans had been on earth. Thus it appears to me the Garden of Eden story was written to show how God had created a perfect human who would be totally obedient to him if they lacked knowledge and did not listen to snakes. Humans being what they are, the struggle has been how to become perfect humans again.
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..