The very first case of incest

Who was the fifth human being on Earth?
The prerequisite of this question is, of course, Christianity's Genesis, the story of Creation. There, God made Adam och Eve, and together they had two sons - Cain and Abel.
Now, the question is, who was the fifth person on Earth? And how did he/she get here?

There is mentioned a third son of Adam's (and Eve's?): Seth, born when Adam was something like 120 years old. And after that, the old geezer went on living, and reproducing himself, for 800 years, having sons and daughters.
But surely something must have happened before the birth of Seth. Surely Cain, the only surviving son of Adam's for a long time, must have had the urge for sex, and thus becoming a father. (There was no such thing as contraceptives then, I imagine.)
And if that is the case, that Cain indeed became a father, then there was only one woman who could be the mother of that child: Eve, thus becoming mother and grandmother at the same time, after having committed, together with Cain, the very first act of incest.

Or is there another explanation?
What if the truth is, as has been suggested, that Eve wasn't the first woman of Adam's? What if God really did create Lilith first; Lilith, a being of such power as to be able to transform herself into... a snake... to get revenge, after being thrown out of Eden...
In that case, maybe Cain didn't need to approach his own mother in order to get some sex. He could make out with Lilith!
And in that case, of course, Lilith would be the second and Eve the third of Gods creatures. And Cain and Abel would be number four and five on Earth.

Now, should I write that story?
Or should I paint the picture?
Or both?
Or neither...