I believe JJ said as much himself that neither of them are Sith, the Sith were a specific order of Dark Side users that use the Darth title, up until Darth Maul appeared the Jedi thought they had been wiped out for a thousand years. Kylo uses the Ren title, the other knights presumably do as well, it is possible Snoke also has a Ren title or something else as their leader.
They did also say at a panel that he wasn't Darth Plagueis, it is just that people didn't believe them. It doesn't surprise me really, from interviews they don't want to do the Sith again, the Sith were defeated and are finally gone after the fall of Palpatine and Vader.
In the end though these are just titles and names, there are other light side users besides the Jedi, their goals seem to be the same, at the end of the day the dark side wants to rule the galaxy.