it's obvious that Lucas feels no particular loyalty to his original plan or the continuity that he already committed to film....
...however, for what it's worth, I found this version of a shooting script for ROTJ. In it, it not only states that Owen is Ben's brother (which is out the window), but it also states that Anakin left home without knowing that Padme was pregnant (which should have been obvious to everyone anyway.) The only reason I bring it up is because I occassionally see speculation about EP III which includes Anakin knowing about the pregnancy before hitting the road. This, of course, doesn't definitively put that idea to bed, as the whole Owen thing attests to, but this was Lucas's (at least Kasdan's) original intention anyway.
Return of the Jedi Shooting Script
Hmm. To protect you both from the Emperor,
you were hidden from your father when you
were born. The Emperor knew, as I did, if
Anakin were to have any offspring, they would
be a threat to him. That is the reason why
your sister remains safely anonymous.
Leia! Leia's my sister.
Your insight serves you well. Bury your
feelings deep down, Luke. They do you credit.
But they could be made to serve the Emperor.
Luke looks into the distance, trying to comprehend all this.
BEN (continuing his narrative)
When your father left, he didn't know your
mother was pregnant. Your mother and I knew
he would find out eventually, but we wanted
to keep you both as safe as possible, for as
long as possible. So I took you to live with
my brother Owen on Tatooine... and your mother
took Leia to live as the daughter of Senator
Organa, on Alderaan.
Luke turns, and settles near Ben to hear the tale.
BEN (attempting to give solace with his words)
The Organa household was high-born and
politically quite powerful in that system.
Leia became a princess by virtue of
lineage... no one knew she'd been adopted, of
course. But it was a title without real
power, since Alderaan had long been a
democracy. Even so, the family continued to
be politically powerful, and Leia, following
in her foster father's path, became a senator
as well. That's not all she became, of
course... she became the leader of her cell
in the Alliance against the corrupt Empire.
And because she had diplomatic immunity, she
was a vital link for getting information to
the Rebel cause. That's what she was doing
when her path crossed yours... for her foster
parents had always told her to contact me on
Tatooine, if her troubles became desperate.
Luke is overwhelmed by the truth, and is suddenly protective of his sister.
But you can't let her get involved now, Ben.
Vader will destroy her.
She hasn't been trained in the ways of the
Jedi the way you have, Luke ... but the Force
is strong with her, as it is with all of your
family. There is no avoiding the battle. You
must face and destroy Vader!