Eve thought Cain was THE ONE (see verse 1). It does seem that Cain was the first human baby; maybe not though. This chapter covers decades, or perhaps even centuries. Adam was 130 years old when Seth was born, so doubtless Cain had dozens of siblings that remain unknown to us (younger ones for sure, maybe older ones too). Obviously Cain had to have married one of his sisters or nieces (no problem there at that time since the gene pool was richer then and there were no cultural norms, precedents or commandments regarding such things… as of yet). Adam celebrated 930 "made-from-dirt-days" and had sons and daughters (Genesis 5:4), so I've got no problem with the old "where did Cain get his wife" question.
To me, the important thing in this passage is the continuous unveiling of God's redemption plan. He had promised Eve earlier that one of her children (however many generations removed) would deal with THE serpent (Genesis 3:15/Revelation 20:2). It seems clear to me that she thought Cain was that promised seed, or at least the father-to-be of that descendant. Yet, perhaps as her two most famous sons grew up, she recognized the reverent spirit in Abel and the rebellious spirit that was in Cain... and maybe decided that Abel was actually the promised seed. Wrong again! Next thing she knew, her "spiritual" son was dead... murdered at the jealous hands of his wicked brother. What now?
The chapter closes with Eve's hope, ("God has appointed me another seed [Seth] INSTEAD of Abel, whom Cain slew" - verse 25), and with a clear picture of God calling out a people for Himself (verse 26). When (in our 3 & 5 to Thrive journey) we get to the Gospel of Luke (a few of years from now), you will see that Seth became the great grandfather (well, great x 70) of THE SEED, Jesus Christ (see Galatians 3:16).
There will be many Cains and Lamechs who live without regard to God's holy name, but there will also always be a remnant of believers who offer to Him an acceptable sacrifice of praise and who, by faith, "call upon the name of the Lord."God always has some people reserved unto Himself! Are you one of them?