Are they step brothers???
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 °)╯╧╧
"If the society says flat chest is justice then big titties is peace" wtf ahahahha this is hilarious