Found: YA Fiction. Princess in Hiding. Plays Harp. 13 Adopted Princesses.

This book is at least 15 years old and features a princess in hiding with a special royal advisor who pretends to be her harp teacher. She eventually makes her way to the palace to relieve the "stand-in fake" princess of her duty and it is revealed that there were actually 13 princesses adopted by the King and Queen before their violent deaths and each was ushered into hiding. Leaders of the revolution are looking for the true princess and have already imprisoned a few that swear they are the real princess.

I believe you are looking for Palace of Mirrors by Margaret Peterson Haddix?

>2 vroni: thank you!

This book has been found!