What was the relationship between Mark and Barnabas?
Mark was the cousin of Barnabas. When Paul did not want to take Mark along, Barnabas took him with him to Cyprus. Colossians 4:10; Acts 15:37-39
What the Bible says
Colossians 4:10
Aristarchus, my fellow prisoner, saluteth you, and Mark, the cousin german of Barnabus, touching whom you have received commandments; if he come unto you, receive him:
Acts 15:39
And there arose a dissension, so that they departed one from another; and Barnabas indeed taking Mark, sailed to Cyprus.
Douay-Rheims Bible (Challoner revision), public domain.