I would argue that Luke and Leia being revealed as siblings in ROTS is much better, dramatically speaking, than having the reveal be in ROTJ, for a couple reasons.

1. What is a better reveal, Obi Wan plainly saying "you have a sister," and then Luke correctly guessing Leia; or Luke being born, and then all of a sudden, twins! Luke has a sibling?!? Padme names is Leia! Luke and Leia are siblings! It just seems more exciting to learn it this way.

2. It would be a nice payoff for the machete ordering, connecting ROTS and ROTJ further and making the whole flashback worth while, as you get new information about the characters you started with.