Biblical scholars seem to agree that Cain married a sister, and that marriages between brothers and sisters were common early on.
It wasn’t forbidden until Leviticus.
It is ghastly for us to think about, but as caveats, remember that people were living hundreds of years then; so brothers and sisters were not necessarily raised together. Also, presumably the progressive genetic damage we are all inheriting was not yet present, so genetic problems were not as likely at all.
It may well be that some biblical scholars agree but it does not add up to what we are told about what Cain did. And have you noticed how closely the names of Cain's genealogy are to those of the Adam's genealogy?
Notice what is written in Genesis 4:14 "..... Behold, Thou has driven me out this day from the face of the earth; and from Thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that everyone that findeth me shall slay me."
Why would Cain be worried about 'everyone' and not his immediate family if only his immediate family were the only peoples on this earth? No other children are recorded by Adam and Eve until after Cain left the area. And I agree we do not know how many other children Adam and Eve had, only the genealogy listed in chapter 5 leading up to Noah.