Actually Banjo is the name here which makes the family seem Australian, as I would assume the parents were inspired by Banjo Paterson:
http://www.wallisandmatilda.com.au/banjo-paterson-biography.shtml
Given that association,
Atticus, Arlo, and Banjo seem to be linked by a sort of combined "liberal politics & folksy lifestyle" vibe that makes it unsurprising that they are brothers.