the emperor was Sith, he was a very old sith lord who had many clones. and despite the death of Vader and the emperor the Sith still exists, as it is a natural counterpoint to the jedi.... not all dark side users are Sith Lords... actualy the Epire had many Darksiders ("Dark Jedi") working for them. they werent Sith though... however they could become and learn to be a sith. much like how not all light siders are Jedi.

but yes, the sith lives on