According to the almighty mos maiorum, the patrician class's relationship to the plebeians was analogous to the power of the pater familias over his wife and children.
Finally, the Roman anthropomorphic gods and goddesses made up a family ruled in the very best Roman sense by the great shining pater familias Jupiter, formerly a lightening bolt.
No wonder the idealized picture of Roman life back in the days of the mos maiorum included a pater familias, who went from the field to the battle field and back as needed.
And " pater" , the Latin version of it. Okay. So then we have: " brother" , if you have a brother, they could be called in a very informal way: " bro" .
" Mater-" means mother in Latin, that is where we get " maternity" from; and " pater-" in Latin means " father" and that is where we get " paternity leave" or " paternity, " rather.