We have no idea how long Adam and Eve resided in the garden before the fall. And we are not told how many children they had.
When Cain was expelled from the garden,
who was he referring to when he feared
they would kill him if he was outside the gates of Eden? Who did God (placed a mark on Cain so they wouldn't kill him) protect him from? Who did he marry? Did one of his sisters get kicked out of Eden too?
How old was Cain when he killed his brother? 16? 66? 106?
Gen.5: 3-4 And he begot Seth, the days of Adam were eight hundred years; and he begot sons and daughters. So all the days that Adam lived were nine hundred and thirty years; and he died.
The Bible says Adam was one hundred and thirty years old when Seth was born. He then lived another eight hundred years. In Gen 3:16
God had promised Eve that he would greatly multiply her conception.
Jewish tradition states that Adam had 33 sons and 23 daughters! Therefore
many people could have existed at the time when Cain killed Abel.
It has been conservatively estimated that
32,000 people could have been alive at that time this event occurred.
Before the fall, before sin entered the bloodstream, Adam and Eve had perfect gene pools, If Cain married a sister, or niece, or great niece, or great great great niece, no imperfections were there to pass on.
It wasn't until the Law, that marriage was defined.
Though Genesis condemns incest, relations between children and parents, it does not forbid a man from marrying his sister or niece. The Bible records other examples of intermarriage within families. Gen 20:12 Abraham married his half-sister Sarah. Ex. 6:20 Moses' father Amram, married his father's sister, his aunt Jochabed.
Then came the Law:
God forbade marriage between the following: mother, father, sister, brother, half-brother, brother's wife, aunt, uncle, stepmother, granddaughter, daughter-in-law, and son-in law. Before that time, it was not forbidden (Leviticus 18:7-17).