in a nutshell, the empress ranks much higher compared to the queen. we can say that the empress is the legal wife or consort whilst the queen is the second wife.
Empress has a higher standing usually first wife Queen has a lower standing, second wife Kings marry multiple people for more political power, alliances with other countries and stuff like that The queen and empress thing is in reference to this comic specifically.
I'm a bit confused, what's the difference between empress and Queen, and are they married to the same person? I think I'm missing some parts.