My understanding of Cains wife is that Adam and Eve had more than two children. Being so, that means if Eve had, say, 50 kids, and those kids bore children, then Cain's wife would have been his second or third cousin.

Or maybe God created humans outside of the garden of Eden, the peoples of other races than white. ( yea, I know that sounds rassist, but they could have been black or brown or yellow)