What if Anakin/Vader didn't know that Padme was pregnant? What if that's the reason that they were able to hide Luke all these years? Vader certainly didn't know that he had a daughter....
"A sister...? Obi-Wan was wise to hide her from me."
Since they were twins, it sort of figures that he didn't know about either of them.
I predict that Anakin will become Vader while Padme's very much alive - but she'll leave him and they'll have no contact for some time (at least 9 months, say!)
But, she will be killed in Episode III - story-wise, that's just how it's got to go.
I also remember Lucas talking about the prequel-trilogy and the difficulty he had in devising the story so that the "I'm your father" bit in Empire would remain a powerful surprise to a new generation who would be able to watch Episodes I to VI in order for the first time. If he's setting that bit up to still be a shocker, then maybe they don't EVER show Padme pregnant - or delivering twins. Or, maybe the don't show Anakin becoming Vader - somehow they "convince" us that Anakin and Vader are not one in the same.
I don't know how he'll pull it off, but unless he throws some clever plot twist in Episode III, there will be a lot of first-time viewers in 3 years watching Star Wars for the first time shouting "Dude, Vader is your father!!" during the first attack on the Death Star.
- FJ