James is writing to the Jewish believers who were scattered abroad , those who believed in Jesus , and his letter is also shown to us as how we should live our lives in Christ...
Many Jewish believers from all the tribes were scattered...
Again, the term twelve tribes never means a called out group from the nation of Israel. It always means, throughout Scripture, the nation of Israel as a whole. You just allow the simple text to determine what is said and not man's commentaries on God's word. You would never come to this conclusion through personal study and comparing Scripture to Scripture.