Thers not anything wrong here?! "After the Galactic Republic fell, the galaxy was ruled by two Dark Lords—Galactic Emperor Darth Sidious and his apprentice, the fallen Jedi Darth Vader—during the ...
Btw. I have the feeling a "Sith Lord" is not that high of a rank within the Galactic Empire governance structure. I remember in Episode IV Grandmoff Tarkin shouting at "Vader" without even using the Lord title and acted as if Vader is his Lieutenant. But maybe that's only for the original idea in "A New Hope". They clearly changed it in the later films.