The Greek in Colossians 4:10 that is translated into English as “cousin” is translated into Banaro as “Barnabas’s younger brother.”
William Butler (see here ) tells this story:
“For ‘cousin,’ Samuel had used the word donghang, the singular form of the word we had used for ‘brothers’ in other places in the book. However, the checking committee rejected the singular form being used in that way. They insisted that a proper kinship term be used. That is where our problem began. There is no Banaro term that means the same as ‘cousin.’ In the Banaro system, all your uncles and aunts are called by the kinship term for ‘father’ and ‘mother.’ Therefore, it is only logical that their children, your first cousins, are referred to by the same term as ‘brother’ and ‘sister.’ And guess what you call their children? ‘Son’ and ‘daughter’! So you see there isn’t any room in the system for cousins, as the English word is used.
“Somewhere in the discussion I remembered that we weren’t translating from English but Greek, so I looked up the Greek word that is translated cousin in English, hoping to find some help. The Greek word is more specific than the English word, specifying a first cousin. Therefore, we needed to use the correct Banaro term for a first cousin: ‘brother.’ Not so hard, eh?
“But in Banaro there is no general kinship term for brother. Age rank is important in the culture so one must specify older brother or younger brother. Considering that Barnabas seemed to take Mark under his wing and Mark’s action in turning back on the journey he started out on with Paul and Barnabas, we decided that Mark was likely younger. He is, therefore, ‘Barnabas’s younger brother.’ You have to realize that when a Banaro person reads this he will not automatically assume that Barnabas and Mark are siblings of the same parents but will consider the wide range of relationships covered by this term in their culture. We will also have a footnote trying to further define the kinship relationship that likely existed between the two men.”
Otherwise, translations differ between translating the Greek into “cousin” or “nephew.” The English King James Version / Authorised Version has sister’s son (which it took over from Tyndale’s translation), the German Luther translation has Neffe (“nephew”) (changed to Vetter or “cousin” by the revision of 1956), many Orthodox translations that followed the Church Slavic translation (first publ. in the 9th century), translated under the leadership of the brothers Cyril and Methodius who grew up speaking Greek in today’s Thessaloniki, as well as the latest Modern Greek versions all have “nephew.” The reason is most likely the shift of meaning of anepsios (ἀνεψιός) between Koine Greek and Modern Greek from “cousin” to “nephew” which is being ignored by Greek-speaking translators. While it’s not clear why Luther and Tyndale also chose a form of “nephew,” it’s possible that they were being taught Greek by (Modern) Greek-speaking who might not have been aware of that shift in meaning as well.
In the Latin tradition, the Latin Vulgate translates this as consobrinus or “the child of a mother’s brother.” Other languages that, like Latin, don’t have a generic term for “cousin” without an indication whether the cousin comes from the mother’s or father’s side, decide differently. Examples of historic translations in Coptic, Classical Syriac, Classical Armenian, or Old Georgian it is translated as “the child of a father’s sibling.” Why Tyndale and, following him, the King James Version (see above) make (an unforced) decision here is not known. (Source: David Clark in The Bible Translator 2014, p. 349fff. )
Seer also cousin.