If everything in the original Bible is true, why is Cain not mentioned once in the genealogy of Adam? If we look into the word superinfertation at the three latest I have seen in medical files is there perhaps a way that Cain could have been born just before Abel without Adam knowing Eve again to conceive Abel? It does mention that Adam did go into Eve or knew her and then Seth was born.
"Now Adam knew Eve his wife, and she conceived and bore Cain, and said, "I have acquired a man from the LORD." Then she bore again, this time his brother Abel. Now Abel was a keeper of sheep, but Cain was a tiller of the ground" (Genesis 4:1-2).
It doesn't require any fanciness to see what happened. Adam and Eve had sex, Eve conceived and eventually had Cain. The Hebrew literally states that Eve then "added to bear" Abel. Most likely what is being described is that Cain and Abel were twins. Cain was born first and then Abel was added to the family. Perhaps that also explains the boys' names. Cain means "acquired" but Abel means "vanity," which seems to hint that he was seen as an extra.
If I asked you to list your ancestors, most likely you would trace your direct lineage back several generations. You wouldn't bother with all the siblings in each generation because they don't directly relate to your own existence. The genealogy that starts in Genesis 5:1 runs through Genesis 6:9. The phrase "these are the records of" or "this is the genealogy of" is the same wording in Hebrew. It is a signature line for whose records are recorded before. The genealogy from Genesis 5:1 to Genesis 6:9 is Noah's. Since he descends from Seth, only Seth is mentioned. He doesn't mention either Cain or Abel directly. He only notes that Adam "had sons and daughters" to let us know that Adam and Eve had more than just the three sons mentioned by name in Adam's records. In every generation there is likely to have been multiple siblings, but only the one that directly leads to Noah is mentioned each time.