In Colossians 4:10, Mark is identified as a relative of whom?

Answer: Barnabas

Paul sends greetings from "Mark the cousin of Barnabas" (KJV: sister's son to Barnabas).

“My fellow prisoner Aristarchus sends you greetings, as does Mark the cousin of Barnabas. You have already received instructions about him: If he comes to you, welcome him.”— Colossians 4:10 (BSB)