Cain's Wife
It is often asked where Cain's Wife came from, but the Bible gives us a perfectly clear solution. We know that Eve is the "Mother of all living," as Scripture clearly states. We also know that Adam and Eve had children for 800 years after the birth of Seth. This isn't to say they didn't have children before, only that they weren't recorded. Cain's wife was a relative, likely a sister. This wasn't a problem at that time, as Sin hadn't taken as much a toll on the gene pool of the human population. God later prohibited incest in scripture.
