What was the relationship between Cain and Abel?
Cain and Abel were brothers. Cain took Abel out into the field and killed him there. Genesis 4:2, 8
What the Bible says
Genesis 4:8
And Cain said to Abel his brother: Let us go forth abroad. And when they were in the field, Cain rose up against his brother Abel, and slew him.
Genesis 4:9
And the Lord said to Cain: Where is thy brother Abel? And he answered, I know not: am I my brother’s keeper?
Douay-Rheims Bible (Challoner revision), public domain.
What the Church teaches
Cain’s murder of his brother Abel reveals the anger and envy in man, from the very beginning.
Catechism 2259 · Catechism of the Catholic Church
Did you know?
In this story, the word “brother” appears seven times. NABRE note on Gn 4, 2