Once upon a time, if you wanted a fairy-tale-style shojo fantasy, you had to settle for books like this.
Thank goodness for progress.
PRINCESS PRINCE (Purinsesu Purinsesu), by Tomoko Taniguchi. First published in 1994 and first published in North America in 2003.
PLOT:
The kingdom of Gemstone is seemingly happy and peaceful, but for the royal twins of the land it is anything but that. When they were born, a prophecy foretold that one of them would have to be raised as a girl to prevent disaster. Ever since, Prince Lawrence (aka "Lori") has had to hide his true identity, even has he pines for his best friend Jenny. Meanwhile, his brother Matthew has to deal with Brandon, a former thief who is convinced that Matthew is the one hiding their true gender. Can their secret survive jealous angel birds, near drownings, and a possible mermaid?