Here is part of an answer that is given on the Answers in Genesis website:
"The simple answer is that Cain married his sister or another close relation, like a niece. This answer may sound revolting for those of us who grew up in societies that have attached a stigma to such an idea, but if we start from Scripture, the answer is clear.
1 Corinthians 15:45 tells us that Adam was “the first man.” Genesis 3:20 states that Eve “was the mother of all the living” (NASB), and Genesis 5:4 reveals that Adam and Eve “had sons and daughters” (besides Cain, Abel, and Seth).
There were no other people on earth as some have claimed. God did not create other people groups from which Cain chose a wife, as we are all made of one blood (Acts 17:26). If He had made others, these people would not have been able to be saved from their sins, since only descendants of Adam can be saved—that’s why it was so important for Jesus to be Adam’s descendant.
Foundations
Why is the church not nearly as effective in reaching the lost as it was only a generation ago? Get the DVD series for an answer to this question and many others.
Doesn’t the Bible forbid marriage between close relations? It does, but the laws against marrying family members were initially given as part of the Mosaic covenant, approximately 2,500 years after God created Adam and Eve. Due in part to genetic mistakes, these laws were necessary to help protect offspring from mutations shared by both parents."
If you are interested to see the full answer, it is here:
https://answersingenesis.org/bible-...tlQa0M-aSHgbNPI4ytl8vFgecVQ93OHCulRphqqMRSXPa