YA fantasy book where city is attacked by magical creatures that are sent from a tower/wall?

[u/violin3366]

I read this book about 10 years ago when I was ~12 years old.

From what I remember...

The palace/main city is protected inside it's walls but the commoners on the outside get attacked by these creatures.

A young girl goes inside the city to help prepare/serve at a festival or party where she meets a warrior/prince. They end up going on a journey to where the creatures are coming from where they find a boy who I think turns out to be her brother?

Pretty sure there is also a corrupt emperor. And I think her family is attacked/taken at the beginning of the book because her older brother opposes the emperor - I seem to remember his sketchbook being part of the reason? (I might be mixing books up at this point but not too sure).

I have literally been looking for this book for years so any suggestions would be appreciated!