Luke purposefully calls Gentiles something other than Adelphoi prior to Acts 15. He doesn't call them brothers until Acts 15!!!
Check this out:
"The use of [adelphoi] in the NT is in continuity with OT usage for members of the people of Israel [footnote 115]....But as von Soden notes, 'There can be no doubt, however, that [adelphos] is one of the religious titles of the people of Israel taken over by the Christian community,[footnote 117]'", pg. 39.
Footnote 117 reads: TDNT I: 145.