For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Rudolph Ferreira
In my opinion the Bible does not tell us everything that happened after or outside Eden,I would say that it is not important where Adam and Eve 's sons got their wives,if it was God would have told us. I think Titus 3 : 9-11 says it best
James Kraft
The bible clearly states that we are the desendents of Adam and Eve. God did allow incest in the beginning because the human race was pure. Many lived for a over 800 years. After the flood we are all he desendents of Noah. Then God gave us a rainbow to promise not to destroy us again by flood.
God does permit things in one generation that he does not in another. During one dispensation God allowed men to have more than one wife. Solomon had many wifes and concubines. God permitted it, but His plan of one man and one woman was His perfect will for us.
We cannot put God in a box so we can handle Him. We cannot understand all His ways. His ways are not our ways and our ways are not His ways. Some things are known only to Him. No one knows and understands the whole bible. Finite man cannot understand the things of God perfectly. The whole bible is true whether we understand it or believe it. It is the revealed will of God.
God works different in different dispensations. After Jesus came salvation is by grace through faith, without works, lest anyman should boast. We can only boast in the cross to save us. Otherwise we could earn our salvation by good works. Salvation cannot be earned because it is a free gift to all who believe that Jesus was God and man and raised from the grave on the third day. It is by simple faith in Jesus finished work on the cross that saves us.
Chris Heier
I think you have it, Amen. We cannot know all, we live by Faith in the Word Incarnate, Jesus Christ. Now we see only a reflection, our mortal sight.
allen anderson
The bible does not say that Cain & Abel were the firstborn children of Adam and Eve, so it's possible that they could have other unnamed children before Cain & Able, and thus Cain married an unnamed sister.
Shirley H
AMEN, Rudolph, James, Chris, and Allen. God tells us everything we need to know. The Bible does not necessarily tell us everything we may want to know. I believe that one day we will be able to have the answer to anything we want to ask. Until then we live by faith, trust the Lord and seek His help.