Like in "Star Wars: Die Rache der Sith" Sith is feminine why?
Der is plural in that case not feminine, and the gender information is automatically lost with the German article.
Applied for individuals
der Sith or
die Sith could be used gender specific:
Der Sith, der meinen Vater getötet hat.
Die Sith, die meinen Sohn geboren hat.
Note the recurring article for these cases of use.
Worth to mention, that "Sith" is used in the Star Wars universe to attribute a group of people, and not a particular individuum('s name).