Marrying Your Sister?

Who did Cain marry? Did Cain marry his sister? Why are these questions relevant to our daily life?

I like to ask questions that aren't normally raised to get you to think about what and why you believe the Bible and God. This question has been raised by sceptics of the authenticity of the Bible saying that Leviticus 18-20 states that it is unlawful that anyone should marry his sister. If Cain married his sister...this contradicts God's law and so the Bible is not true! Wrong!!!
Many Christians try to explain that there might have been another race of created people but this is contradicting to the truth that we are all descendants of Adam (Gen 2:7, 1 Cor. 15:21-22). The truth is that yes, Cain DID marry a sister! It is not clear which one or exactly when the marriage was but it is true. Adam and Eve had a total of 33 sons and 23 daughters! for the human race to survive they needed to marry their own relatives and reproduce. When you think about it, we are all brothers and sisters, we are all from "one blood"
So what about that law? My answer to people who raise such a stupid question is get your timeline straight! The law of Moses wasn't written yet during the period of creation and the fall. Abraham even married his half-sister Sarah (Gen. 20:12) and God blessed his life and made the covenant that a great nation will rise from his seed! The laws were not given until about 400 years later with Moses.
So when you hear someone discrediting the Bible as truth because of issues like this...don't try to make up some answer to fit, read the Bible, study the history, and find the truth. "always be ready to give an answer to everyone who asks you a reason for the hope in you" 1 Peter 3:15