For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
From the conversation between God and Cain after his murder, it was evident, that there were already other people than just Adam, Eve or Seth at that time.
Please read Leviticus 18 verses 1-18.
It only became incest (and a sin) after God gave His Law (Commandments) to Moses and the Children of Israel.
Yes Cain murdered Abel his brother (1 John 3:12 NLT " Do not be like Cain, who belonged to the evil one and murdered his brother. And why did he murder him? Because his own actions were evil and his brother’s were righteous.").
The Genesis account is corroborated by Paul - [Act 17:26 KJV] 26 And hath made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds of their habitation;" There were no other humans/humanoids existing before the creation of Adam and Eve, because the bible states -[Rom 5:12 ESV] 12 Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned--". No death before sin, no sin before Adam therefore no humans living and dying before Adam! Simple!
if all men came from Adam and Eve, then how were there others East of Eden in the Land of Nod? Did Cain marry and sleep with his sister? Why did he have to go to Nod to "find" his wife? I do not believe that we "all" came from two people. I believe that God created Adam and Eve as a way to purify the Earth, but Satan poisoned that. Nothing in the Bible is simple to understand....if it was, then every person would believe it or understand it. There would be no war and no separate faiths. Right?