This is the most logical theory I could come up with:
Anakin is "betrayed" by Obi-Wan when Obi-Wan is ordered by the Jedi council to expell Anakin from the Order after they discover his hidden marraige to Padme. Anakin resents Obi-Wan for this and thinks that he was working against him. The Jedi discover this information when it is revealed that Padme is pregnant with a child.
Padme who feels responsible for Anakin's expulsion, is consoled by Obi-Wan who tries to comfort her. Anakin sees this and turns to the only person who he can trust. Since the Jedi have betrayed him, his best friend, and his wife, he forsakes his love for Padme, his friendship for Obi-Wan, and his vows to the Jedi Order.
He then turns to Palpatine, the only man who he can believe in. Meanwhile Palpatine has orchestrated all of this and implicates those closest to Anakin in an attempt to lure him to the Dark Side. Palpatine sees that Anakin is powerful (Ep.II) and wants him to destroy the Jedi- the last thing standing in his way.
The Jedi see that Anakin has aligned himself with Palpatine, who has just declared martial law in the entire galaxy, and once secure in his office of power, declares himself Emperor.
Obi-Wan sees what Anakin has become, and tries desperately to warn Anakin of the Emperor's plans that led him to believe that he was betrayed.
Anakin will not listen, nor budge, and soon their arguing turns to battle, and they fight a furious lightsaber duel. Anakin falls into the molten pit, and Obi-Wan fears that he is dead. When Obi-Wan leaves, Anakin emerges and gives himself completely to the Dark Side.
Once the Emperor has rescued the former Jedi from certain death, Anakin, now Darth Vader, swears full and total allegiance to the Empire, and begins his hunt for the remaining Jedi. He helps the Empire Destroy the Temple and all the Jedi in it, but he does not know that Obi-wan and Yoda have escaped.
Obi-Wan hides Padme and is shocked to learn that she has twins ( the reason why Vader only knows of Luke, and not Leia until ROTJ) They then decide to split the twins to keep them safe from the Emperor, and Vader.
Padme is escorted to Alderaan with Bail Organa and his royal family, where she dies broken hearted, and Leia is given to the care of the house of Organa. (Why Leia remembers her mother slightly-"Images really Feelings"-Leia. If they start training Jedi as infants as we now know from the prequels, then it is entirely possible that she would have some vague recollection of her mother.) The droids memories are wiped clean of any connections to the dicsussed persons. (Why the droids don't remember Obi-Wan or Tatooinne)
Meanwhile Obi-Wan escorts Yoda to Dagobah with the infant Luke to hide the Jedi Master in the swamps. (Thus why Luke finds the planet familiar in ESB) Obi-Wan then travels to Tatooinne, to hide and gives the infant to the Lars to raise. He goes into hiding and changes his name to Ben Kenobi.
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