The kingdom of Myrilia was ruled by the royal family, Estatet. But there were also two Archduke families and three Duke families who also had the authority to sit on the throne of the kingdom. As this world had real gods present, the complete change of dynasty by some outside usurper was impossible here.
But that doesn't mean the ruling family can't change. Throughout history it had been seen that if a single family had a complete monopoly over the ruling power then that nation most of the time either had given the worst possible life to its regular population or had plunged the realm into endless wars. That was why, most of the nation under god's protection had more than one family with the authority to sit on the throne.
