Well, the obvious answer is that Lucas didn't really have Luke and Leia's early life planned out when he wrote Jedi. He sort of vaguely figured Leia's mother took her away to Alderaan while Obi-Wan took Luke away to Tatooine, and didn't think too hard about why. And at the time he probably figured Vader never even knew she was pregnant, which is why he never searched for the children until he encountered Luke.

When he wrote the prequels, he had to paper over all that so that Padme could be pregnant and have the babies just as Anakin becomes Vader instead of 9 months later, which would have been less dramatic.

Furthermore, he didn't even think of making Luke and Leia siblings until Jedi and only did it as a way of cleaning up the love triangle to keep Luke from looking like a loser.

And lastly, there is no Santa Claus, and your whole childhood is a lie! Mwuuhahahaha!