Maybe half brothers...
Half brothers but none of the two men is the dad of the current king. The right full crown prince is not him but Arthur that's why he despise him
Spoiler....
They are full brother and the sister is the one from the duke (half siblings). Based on spoiler i read in NU forum, the older bro thought he is the only one from the king but apparently little bro too. Also, only king descendants can wield the magic which is the little bro(arthur). Older bro seems to inherit his father jealousy, obsessive character smh.(╯°Д °)╯╧╧
Thanks guys
Are they step brothers???