If Cain had Married his Sister, Is it not an Incest?

Q: An atheist commented that the God DOESN'T exist and I asked why they believe that, they responded that God only made Adam and created Eve who had 2 children. Cain and Abel and Cain killed Abel so where did other people come from to continue the generation. The other comment was even if there were other children from Adam and Eve then it was incest. Please help me with your response cos I never questioned it before and now I feel I have to respond in a way that will change the heart of an atheist back to their Creator. God bless.

A: First of all you cannot change an atheist only God can. But we can answer some of their question based on the word of God. First of all after Cain and Abel were born, Adam had sons and daughters (Gen 5:3-4). Sure enough Cain married his sister and the same thing must have happened to other siblings of Adam (Gen 4:17). But when people say it as incest, it is now. Who sets the guide lines? God. After creation God said multiply and replenish and created just one family, so there is no way for them to import a woman from anywhere. God sets the rules and he has the rights and reasons to allow something that we cannot understand with out human mind. When God created the DNA of people it was not so corrupted with various infirmities as like now, but then it was pure and the result would be their offspring would not be born with physical infirmities as of now. But as of now as in the time of the Patriarch's and Moses, an inline or close cousin marriage may result in magnifying a disease which will result in disabled children's born to them (Lev 18:11). Then it was right in God's wisdom, now it is not so.

I hope that will do for our natural reasoning. Pray for the atheist so that his spiritual eyes may get opened, as a result of it he might see that JESUS Christ is the only begotten Son of God who can save him from sin, death and eternal judgment ahead. Blessings.....