You display a rather dubious grasp of logic and consistency of thought. Let's examine what you wrote, shall we?
Premise 1 (yours): "it's proven enough that were here from two people."
Premise 2 (yours): "Adam and Eve never had any daughters."
Conclusion 1 (yours): Cain never married his sister (by Premise 2 alone)
And there, inexplicably, you left it. Now, then,
Conclusion 2 (mine): If Cain never married his sister, then either Cain never married anybody (and therefore the human race comes rapidly to an end) or there was some other, unmentioned female human who Cain did marry, and we are here not from two, but from a minimum of 3 people.
Premise 3 (mine): This does not seem to solve the problem of genetic drift through intermarriage, however, since whomever was Cain's offspring (in this case, we must suppose, Enoch) will have the same problem -- who to marry? Well, it's either got be Eve, or his own mother -- or yet another created female not related to Adam or Eve or Cain.
Conclusion 3 (mine): either God went on a "female creation spree" for some time which every biblical writer has been at pains to hush up, since it's never written about, or the whole premise was nuts before we started. I'm choosing the second option.