What was the relationship between Rachel and Benjamin?
Rachel was the mother of Benjamin. She died giving birth to him; she named him Ben-oni, and Jacob named him Benjamin. Genesis 35:16-19
What the Bible says
Genesis 35:18
And when her soul was departing for pain, and death was now at hand, she called the name of her son Benoni, that is, The son of my pain: but his father called him Benjamin, that is, The son of the right hand.
Genesis 35:19
So Rachel died, and was buried in the highway that leadeth to Ephrata, this is Bethlehem.
Douay-Rheims Bible (Challoner revision), public domain.