In Phantom Menace, there are only two Sith: a master, and an Apprentice. Darth Maul gets the chop, leaving, presumably, a vacancy.
In Attack of the Clones, Saruman is an evil Jedi, throwing around Emperor-style bolts of lightening which even Darth Maul showed no signs of doing.
Do we conclude:
1: That Saruman has replaced Maul as Palpatine's apprentice, having turned to the Dark Side at some point in the ten years since Phantom Menace?
2: That there are other societies of evil Jedi apart from the Sith, and Saruman belongs to one of those?
3: That Yoda was mistaken, and there are sometimes three Sith at once?
4: That Yoda's remark at the end of Phantom Menace (that he doesn't know whether Darth Maul was master or apprentice) foreshadows a clever twist: Maul was the master, Palpatine the apprentice, and Saruman was a sort of apprentice in waiting, ready to take on the role when the master died and everyone moved up once?
5: That Lucas is making it up as he goes along?
I always feel that a Doctor Who scriptwriter is not really trying as soon as the expression "evil renegade Time Lord" is mentioned. I feel that "previously un-mentioned tribe of evil Jedi" betrays a similar kind of shorthand laziness.
In Empire Strikes Back, Vader knows of the existence of "the Son of Skywalker"; but in Return of the Jedi, he is surprised to learn that he also has a daughter. It is even possible that Obi-Wan does not know of the existence of Leia,: in Empire Strikes Back he does not know of the existence of Yoda's "other".
In A New Hope we are told that Anakin-Vader "wanted Luke to have his lightsabre when he was old enough". We are also told that when Vader left Kenobi, he was still only a learner.
Thus, we require a situation in which --
A: A still-good, still-apprentice Anakin has asked Obi-Wan to pass his lightsabre onto his son, but doesn't know that he has a daughter.
B: Although Luke is fostered with Anakin's stepbrother on his home-planet, and although he is living openly under his real name, and although Vader knows of his son's existence, he never goes after him there.
C: Leia remembers her mother, but Luke has no memory of his
To pull all this together, I think we are going to have to have some enormous hand-wave. I think that Vader is going to lose his memory. Or, perhaps his conversion to the Dark Side will be conceived of as a Jekyl and Hyde transformation, which leaves him perceiving Anakin Skywalker as an entirely separate person. This might even make Kenobi's claim that Anakin's transformation meant that Luke's father was destroyed literally true. I predict that Episode III will go something like this:
1: Amidala gets pregnant. This is kept secret from everyone; no-one outside Naboo even knows they are married.
2: Anakin says goodbye to his pregnant wife, and goes to fight in the Clone Wars with Obi Wan, who for some reason is being referred to as General Kenobi, and "serving" Bail Organa.
3: Anakin sees that the galaxy is collapsing into chaos. He still believes in his "benevolent dictatorship" theory as expressed to Amidala in Attack of the Clones. He states that he wants to help Palpatine bring order to the galaxy. Obi-Wan forbids him. Anakin resigns from the Jedi order, and, still a learner leaves Obi Wan. At this point, of course, he doesn't know that Palpatine moonlights as Darth Sideous.
4: Palpatine, tells him about the power of the Dark Side. He tells him that if he becomes a Sith he will be the greatest Jedi ever, all-powerful, and capable of even stopping people dying. He probably takes him to a high place and breaks out in an allegory. At any rate, Anakin gives way and starts to undergo Jekyl and Hyde transformations, from Anakin to Vader and back again.
5: In his Vader form, he helps Palpatine hunt down the Jedi. But in a lucid moment, he warns Amidala and tells her to hide his child with his mother's husband on Tatooine, where he will be overlooked by Palpatine.
6: Little does he know that he does not just have a son, but twins!
7: Amidala does as she is told. Yoda escorts the new-born boy to Tatooine, stopping off briefly on Dagobah on the way. ("There's something familiar about this place!") But she takes the girl to Alderaan where Baal Organa will protect them.
8: The trade federation trace Amidala to Alderaan. They come to kill her. She gives her child to Baal to take care of. The trade federation assumes that Leia is Baal's kid, and no-one sees any need to disabuse them of this misconception. The trade federation kills Amidala. She recites some bad dialogue, and expires.
9: Palpatine doesn't think to look for Luke on Tatooine as obviously no-one would be stupid enough to hide Vader's son on Vader's home planet. Vader, as long as he is Vader (and not Anakin) doesn't know where is son is. But years later, when he finds out that the name of the young pilot who was strong in the Force turns out to be "Skywalker", the long buried memories of his previous identity start to return.
10: Vader is sent by Palpatine to kill Obi-Wan. There is a big fight. At the last minute, Obi-Wan says something like "By the way, did you know you now have a son?". This makes Vader turn back into Anakin for a second. He hands his lightsaber to Obi-Wan and says "Could you make sure my son gets this", and then jumps into the fiery pit.
12: Obi-Wan, assuming that Vader is dead, goes to Tatooine, partly because it is remote and partly because he gets to be near the Anakin's child. Presumably, it is a few years before he finds out that Vader survived and has acquired a suit of armour and a breath mask.
13: All the other Jedi are killed.
14: Twenty years pass, and Star Wars begins.
I trust that is all perfectly clear.