Good question and any answer is strictly speculation.
:
Genesis says:
5 This [is] an account of the births of Adam: In the day of God's preparing man, in the likeness of God He hath made him;
2 a male and a female He hath prepared them, and He blesseth them, and calleth their name Man, in the day of their being prepared.
3 And Adam liveth an hundred and thirty years, and begetteth [a son] in his likeness, according to his image, and calleth his name Seth.
4 And the days of Adam after his begetting Seth are eight hundred years, and he begetteth sons and daughters.
5 And all the days of Adam which he lived are nine hundred and thirty years, and he dieth.
6 And Seth liveth an hundred and five years, and begetteth Enos.
7 And Seth liveth after his begetting Enos eight hundred and seven years, and begetteth sons and daughters.
8 And all the days of Seth are nine hundred and twelve years, and he dieth.
You will see in the above, only Seth begetteth daughters.
Did Adam and Eve have daughters, not until after Seth was born..
Now Adam was 130 years old before Seth was born , so over a century of history is unknown.
Now Genesis 4:14 says
14 lo, Thou hast driven me to-day from off the face of the ground, and from Thy face I am hid; and I have been a wanderer, even a trembling one, in the earth, and it hath been -- every one finding me doth slay me.
Seth is not around yet, or was he?
We do not know how old Cain got or when this happened, but Cain says - every one -- not my Father-- so some how there got to be a lot of people while at the same time Genesis 5 does mention later off-spring of Adam having sons and daughters, with few specifics leading to Noah, who was what , when and how is pure speculation.
The history of specific people in the Bible is there to lead one to Jesus; 99.99999 percent of history is simply unknown.
One thing to think of; if dudes were marrying their sisters, well, Eve came from Adam's rib, so he was screwing himself really.