Tearmoon Empire: Volume 1 by Nozomu Mochitsuki

Crown Prince Sion is prophesied to be fatally poisoned by his younger brother, Echard. Donning a precious keepsake dress made by her mother, ex-selfish-princess Mia throws open the doors to the ballroom, prepared to confront this swirling conspiracy.

After considerable (by Mia standards) thought, she devises a brilliant (again, by Mia standards) scheme. Time to keep Sion dancing so he doesn’t have a chance to drink anything! Unsurprisingly, her plan falls flat almost immediately, as the myriad of players in this game of power won’t just sit idly by...
Up against Sion’s burden of justice, Echard’s inferiority complex, and Sunkland’s complicated political background, can Mia lead everyone toward a better, brighter future?