The answer is: Cousin

Colossians 4:10 - Some older versions have translated as sister's son (or nephew) but the Greek word here 'anepseeos' is the word for cousin and is translated as such in modern translations.

So, let’s talk about John Mark and his family tie to Barnabas—what a relationship dynamic! John Mark is described in Colossians 4:10 as Barnabas's cousin, showing thought that familial connections often played significant roles in the early church. For early Christians, family meant support and companionship, adding a layer of connection to their missionary endeavors.

John Mark is featured prominently in the adventures of Paul and Barnabas, particularly in Acts. Interestingly, there's a bit of drama here; during their first missionary journey, Mark bailed out on them partway through, which inspired a heated disagreement later on between Paul and Barnabas over whether to continue their partnership with him. This incident offers a reminder that interpersonal disagreements existed even among notable leaders of the faith.

And here’s a fun bit of trivia: Mark eventually became a prominent figure in the early church himself. He is credited with writing the Gospel of Mark, traditionally viewed as the earliest gospel. How's that for a comeback? His narrative showcases Jesus’s acts and power, focusing on action—much like he and Barnabas must have showcased in their own missionary journeys.

Thus, in unfolding the intertwining stories of these early church figures, we gain insights not only into their relationships but also into a broader narrative of redemption, second chances, and the vital role of community in disseminating their shared message.