bet if they had been made in that order the fact that Luke and Leia are Darth Vader's children would have been downplayed considerably in RotS so it could have more of an impact in ESB.
Maybe even don't show at all the anakin/Vader transformation. Anakin dies in episode 3, then there is a new bad guy in episode 4, periode.
That way the secret is not only kept for Luke and Leia, but also for the audience.