What was the relationship between Abel and Seth?
Abel and Seth were sons of Adam and Eve. Seth was born after Abel’s death. Genesis 4:2, 25
What the Bible says
Genesis 4:2
And again she brought forth his brother Abel. And Abel was a shepherd, and Cain a husbandman.
Genesis 4:25
Adam also knew his wife again: and she brought forth a son, and called his name Seth, saying: God hath given me another seed, for Abel whom Cain slew.
Douay-Rheims Bible (Challoner revision), public domain.