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.

When we delve into the relationship between John Mark and Barnabas in the New Testament, we unveil an intriguing family tie that reflects the camaraderie of early Christian missions. Understanding them as cousins expands our appreciation of their partnership in spreading the Christian message.

Barnabas, whose name means “son of encouragement,” is a significant figure in the Pauline narrative. He hailed from Cyprus, which adds another layer of geographical significance to their journeys, showcasing the diverse origins of early church leaders. In Acts, Barnabas plays a crucial role in vouching for Saul of Tarsus—who later becomes the Apostle Paul—after Paul's transformative experience on the road to Damascus. This act not only highlights Barnabas's supportive nature but also creates a compelling picture of unity among believers, something sorely needed in an era when the burgeoning faith faced persecution and skepticism from both Jews and Gentiles.

In addition, the relationship hints at the challenges and disagreements that can occur even among family and friends in ministry. When Paul and Barnabas went separate ways over a disagreement concerning John Mark, it emphasized that even leaders could have conflicts, but also that reconciliation and new paths could emerge—as Mark presumably matured and returned to serve alongside Paul later.

Overall, this familial bond illustrates the dynamic interplay between personal relationships and the collective mission of early Christianity in spreading love and teachings across diverse lands.