I've always thought they were twins, but I didn't want to say they were twins, and then have someone prove me wrong, cuz I realized I wasn't sure. Thanks for clearing that up for me.
I'm pretty sure they were hidden from Anakin before they were born, without him even knowing he had a child. I mean, if he knew they would have hunted him down much sooner. The Emperor knew about the "son of Skywalker" being really powerful, so if he knew earlier he would have hunted him down using his Emperor powers/resources.
Obi-Wan was sent to Tatooine to watch over Luke from afar, and when the time came convince him to learn the ways of the force (he tries to convince him to come to Alderan). Yoda no doubt told him to do that. Yoda I think was the mastermind behind it all. He knew who Luke has the potential to be, so he sent Obi-Wan to watch over him. But Yoda also knew how dangerous it was to keep the twins together and he also made sure no one knew they were siblings, giving Leia to Bail Organa. I really thing the only one who knew Luke and Leia were twins was Yoda and the only one who knew Luke was Vader's son was Yoda and Obi-Wan, maybe R2. R2 seemed to know alot about Obi-Wan.
Good point about Leia not necessarily talking about Padme in ROTJ. I never thought of that. She could just be talking about Bail's wife thinking that was her real mother, not knowing who her REAL mother was. It actually makes sense, cuz that would explain why she has memory of her mother and Luke doesn't.
Anyway, it'll be interesting to see how it all plays out in Ep3. I hope George actually WATCHES the original trilogy before editting it, because we all know how many plot holes and continuance problems have cropped up since Ep1 and 2 came out....