As already mentioned, "Sith" is an old concept. Vader is referred to as a Dark Lord of the Sith in the novel among other places. The "Knights of the Sith" appear in the first rough draft written in 1974. The character of Darth Vader becomes a "Black Knight of the Sith" in the second draft in 1975 before finally ending up a "Dark Lord of the Sith" in the third draft.

The term "padawan" has even older origins. It appears in the very first line of Lucas' original story treatment from 1973 along with the name Mace Windu.

But I believe the two Sith rule is strictly a prequel concept.