If Adam and Eve are the first humans, and they had two sons Cain as Abel, where did Cains wife come from?

"And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters"
-- Genesis 5:4

There are plenty of holes in the story (it is from the holy Bible after all) but where the wives come from isn't one of them. They married their own sisters. Is this the middle east or Texas?