Just throwing wild guesses here:
- Zeke is definitely the emperor.
- Her older sister was in the café, so there is a chance she's cozying up to the prince, possibly working with him to arrange the dissolution of the engagement.
- If point 2 is correct then there are two options.
Option number A (the boring cliché): The sister hates her and is responsible for her assassination. Since the prince is an idiot, it would be an easy way to cause a war that her country would win.
Option number B: She's doing this to save her little sis and bring her home because she secretly cares about her.
-There is also the weird face in the window, probably an enemy.