one important reason why kings would have a harem is mainly because almost every past king believed themselves to be the son of god, chosen one, god reincarnated, blessed one...ect even the european medival kings would often cite that they were chosen by god or whatever and so people would be like omg nooo king must have tons of children, cannot let the bloodline die off! also monogamy was something weird back then so nobody really bothered with trying to change it as for the killing of siblings, well, when the throne is at stake.....there will always be one or two who will kill for that..esp since their mother will be like MY SON MUS BE KING!