Right after a king dies for a few years, the new kings brothers stay around as princes even though any pre-existing new-kings heirs may be "of age". This is in case you have a sick king who dies early on in office. Last time that happened to me, my new king dies of a disease or something and his brother was "elected" even though he had sons. Kinda wierd - but i think i got it. Eventually, the kings brothers just become royal nobility