What was the relationship between Mark and Peter?
When he got out of prison, Peter went to the house of Mark’s mother. In his first letter, Peter calls him “Mark, my son.” Acts 12:12; 1 Peter 5:13
What the Bible says
Acts 12:12
And considering, he came to the house of Mary the mother of John, who was surnamed Mark, where many were gathered together and praying.
1 Peter 5:13
The church that is in Babylon, elected together with you, saluteth you: and so doth my son Mark.
Douay-Rheims Bible (Challoner revision), public domain.