The Queen of Morkaia

It is written that the firstborn must take their seat on the throne in the case of death among the king or queen. But what happens if you are twins?
After the sudden death of their mother, twins Cassius and Ivy must battle for the throne in order to see who will rule over Morkaia. But when they must put their differences aside during the war against the kingdom's enemies, they may lose more than just the throne in battle.