Well, I suppose it's possible, though extremely unlikely, that there was a daughter born before the murder of Abel without it being recorded. It does seem fairly clear to me, however, that Cain is forever separated from his family by his sentence to be "a fugitive and a wanderer on the earth", so he would not have been able to take one of his sisters to wife. I do agree that Cain sleeping with Eve is the least plausible explanation.
Oh, yes, I know that. Sorry to be unclear. I just meant that Cain's offspring (the "wicked people") and Seth's offspring present different problems that have to be dealt with separately.All wicked people are descended from Adam.