So is "Ren" the new "Sith"? I hope that wasn't to avoid using the term Sith, since the name did exist long before the prequels.
Kylo Ren isn't his real name, but rather a title. He also isn’t a Sith. He’s a member of an organization known as “The Knights Of Ren" and he is allied with the First Order. Also, his lightsaber isn't a relic - it is something he put together himself.
While the term "Sith" did exist before the PT, I don't believe it was formally used in the OT. It seems like TFA is going back to the roots of the series.
Although the word Sith was never specifically used in the actual moved, it was a part of the official canon back when they were making the movies.
"Two metres tall. Bipedal. Flowing black robes trailing from the figure and a face forever masked by a functional if bizarre black metal breath screen - a Dark Lord of the Sith was an awesome, threatening shape as it strode through the corridors of the rebel ship."
- p. 11 of the SW novelization from 1976.
"Towering above them in his black cloak and concealing black headgear, Darth Vader, Dark Lord of the Sith, entered the main control deck, and the men around him fell silent."
- p. 42 of the ESB novelization from 1980.
"...and finally Darth Vader, Lord of the Sith, emerged from the void."
- p. 4 of the ROTJ novelization in 1983.
This is how Vader was introduced in all of the three original novelizations. Always a variation of the same phrase; "Dark Lord of the Sith."
Did the novels ever go into any detail about the Sith?
Not that I can remember, and frankly I'm too lazy to re-read the three novels again just to find out.
I literally just scanned the first chapters, looking for the word Sith, and wrote them down. It's been a few years since I read them.
However, from what I remember, the answer is no, I can only remember the term being used in relation to Vader and his "dark" ways. The ESB book definitely gave Vader some very dramatic descriptions, always describing him as a "Dark Lord" and even making a reference to him trying to steal Luke's soul during their fight on Bespin.
However scanning through the end of the ROTJ novel I can't find any references to Palpatine being a Sith. He is however referred to as "the evil one," and the novel gives him some pretty funky descriptions, such as; "his dark majesty," talking about his "wizardry," and describing his face as having "death's grin" and being "lined by holocaust." So the book is definitely trying to sell the character as a sort of tyrannical evil wizard.
When the OT movies and their novelizations were released, I figured that "the Sith" was Vader's Stormtrooper legion, and that Vader, being this legion's leader, was its "Dark Lord." Just like a "Grand Moff" was an Imperial Governor, a "Dark Lord" was an Imperial General. I definitely wasn't expecting "the Sith" to be a two-man dark jedi space illuminati organization who's members backstab each other for no reason. I thought that Vader and The Emperor being the only dark side users was because they weren't part of any order of Force-users, not because of some Rule of Two. I thought that Vader and The Emperor plotting against each other was just because of their personalities, not because of some ancient tradition.
Here's hoping that the Knights of Ren don't backstab each other.
I'm pretty sure that the whole Sith thing was something that Lucas had planned from the beginning. Sure, the whole "rule of two" thing is something he added in the PT, but the term Sith always referred to an evil version of the Jedi order. Heck even the rough draft of the ANH script had several members of the "Knights of the Sith" in it. What's interesting is that the way the Sith were described in the rough draft was actually more similar to the Knights of Ren. Not so much evil, but affiliated with the Empire.
However the OT and the novelizations put a lot of emphasis on the whole Dark Lord thing, and Palpatine being pure evil. Heck just look at (and listen to) his death in ROTJ, it's something you'd expect from Sauron in LOTR. I think the Sith were always meant to be sort of evil wizard cultists.
The thing that I find weird however, that is when taking the EU into account, is how little both Vader and even to some degree Palpatine act as Sith. They're more like Dark Jedi's; hording power for themselves and refusing to join any group other than their own. So Palpatine is more of a disobedient Sith, he follows their evil and mystical ways, but refuses to share it with anyone else, and is quite the backstabber. He's hardly a Sith Knight, or part of any cult, yet he does embody everything about it.