When I did my extremely rough cut of ROTJ the main point of the edit or what I deemed to be the lynch pin of it was that Luke new and tried to keep it secret she was his sister but failed because of his feelings. It was also about his worries of losing the ones he loved.

Breakdown

1.Ben tells Luke Leia is his sister and to keep it secret as this knowledge could be used against him.

2.When Luke is on the Bridge in Ewok village luke does not reveal that leia is his sister to her and does what Obi-wan told him to do.

3.When Luke was hiding from Vader Leia got shot in the arm outside the bunker... Lukes feelings betrayed both him and leia. Vader learns she is lukes sister before leia knows it.

4. When Vader dies it has more emotional impact that Vader tells Luke "tell your sister you were right about me" it works because he already told Leia Vader is his father but Leia does not realise Vader is her father too at this point.

5. When Luke returns to Ewok village they are wearing the same clothes so it is possible for Luke to reveal to Leia that She is his sister as his father told him to although Luke now says "The force run's strong in my family... I have it and.... My sister has it."

Theres nothing wrong with her being lukes sister, it was just not executed very well... I thought this improved it...Although it is tricky to get right at Ewok village