Where did Cain's wife come from?

Cain's wife is not named in the Bible, so it should not be a problem. Genesis 5:4 says Adam and Eve had other sons and daughters, so there were many more children than Cain, Abel and Seth; they could have had a dozen for all we know. At that time there was no problem marrying a sister, because there were no genetic defects yet, and to the first humans it would carry no shame; they knew they had to build the race and this was how. Cain, though, is not in the genealogy, which runs from Adam to Seth to Enosh, and we do not even know that he married, since his punishment was to live as a vagabond by himself.