What was the relationship between Eve and Seth?
Eve was the mother of Seth. When he was born, Eve said that God had given her another son in place of Abel. Genesis 4:25
What the Bible says
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.