My plot at this stage needs refining, but I was thinking along the lines of the song - Hoist the Colors. "The King and his men stole the Queen from her bed and bound her in her bones..."
The King of the Seven Seas - head of the brethren court was ambushed by a King of the Land and was murdered along with his men. The moment the King died, his daughter - Princess of the Seven Seas became Queen but was taken by the King of the Land (stole the Queen from her bed and bound her in her bones). She was perhaps young so she has barely a memory of this.
Due to this kidnapping, Pirates of the Brethren Court made the song and it was only known throughout the seas.
The "Queen" or Princess grew up on the land, thinking the King was her real father, but as she grew older - she found out the truth on who she really was and kept it a secret. With the King dying, she became Queen and began taking over lands of other Kingdoms as when she was to reveal herself..she would have the seas as well as significant land.
Plot .... perhaps she and the General of the army have a love affair, or perhaps the General who is taking over these countries for her is a Pirate which isn't known ..but is rumored throughout the Kingdoms Court. He ends up finding out as she knows the secret song..blah blah blah.