Here are a couple of options, depending on what you're looking for:
If you want a true rhyme:
* Thin / win - This is a perfect rhyme, but it doesn't really relate to the concept of "empress."
If you want a near rhyme or a word with similar sound:
* Thin / spin - This is a near rhyme, and could be used to describe a slim, graceful empress.
* Thin / queen - This is a near rhyme that works with the idea of a royal figure, though it doesn't have the same sound as "empress."
Ultimately, the best option depends on the context of your poem or writing.