There are two Greek words that mean cousin found in the New Testament books. Anepsios is found only once and is defined as cousin or nephew (translated sister's son in KJV). Suggenes and its cousins are found twelve times (nice round number) and are translated in the KJV as kin, cousin, cousins, kinsfolk, kinsman, and kinsmen.
The Apostle Paul had a lot of them, as evidenced in two Catholic chapters of Romans, Sixteen and Nine.
Greet Andronicus and Junia, my kinsmen...Rms 16:7
Greet Herodian, my kinsman. Rms 16:11
Timothy, my fellow worker greets you, and Lucius and Jason and Sosipatar, my kinsmen. Rms 16:21
So you're ok with how Paul identifies these individuals by name and how he views them.
Yet when we go to Galatians 1:19 and Paul calls James, the Lord's brother, you don't buy it.
εἰ μὴ Ἰάκωβον τὸν ἀδελφὸν τοῦ κυρίου
if not James, the brother of the Lord
τοῦ κυρίου is genitive case indicating possession.
James is the Lord's brother.
The catholic rules of Bible interpretation defy any accepted standard.