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.

Isn't family dynamics in the Bible fascinating? John Mark, identified as Barnabas's cousin, plays a crucial role in the early church alongside heavyweights like Paul and Barnabas. The term used in Colossians gives us a little twist—while some early translations said "sister's son," it’s clear the Greek is pointing to a cousin relationship. This detail casts light on how familial ties shaped the missionary endeavors of that strikingly dynamic group.

Imagine being part of the early church, bustling with energy! With Barnabas being known as “the son of encouragement,” it’s no surprise he took John Mark under his wing. This kind of mentorship wasn’t unique to them; it reflects the broader practice found throughout biblical texts, where familial and mentorship bonds were integral for support.

It's also intriguing how John Mark later flourished on his own, composing the Gospel of Mark that aimed to relay the narrative of Jesus’s life in a straightforward manner—almost as though he were an early journalist getting the story right! His reflections on the highs and lows of the discipleship thinly overlap with his early quarrelsome experiences with Paul (Acts 15:37-39), illustrating the complexity of conflicts and later reconciliation even in their passionate mission work.

So, while it’s easy to overlook relationships like that of Barnabas and John Mark, they were vital threads in the colorful tapestry that chronicles the burgeoning church and the spread of early Christian messages.