You're looking at this from the point of view of the "finished" saga but to understand the actual MASSIVE ripple affects this had you need to look at it from the point of view of the story development in 1980 and then 1983. You also need to understand that thinking that the latest version of George's "Revisionist history" on the story of SW is the "truth" really is pure, provable ... NONSENSE. For him to say that there was ALWAYS only supposed to be 6 movies and that the story was ALWAYS supposed to be the "Tragedy of Darth Vader", well the available amount of data to show this to indeed be nonsense, most of it from his own mouth, is overwhelming.
First, lets go back to 1977. Do you realize that when what we now know as ep IV was made Darth Vader and Anakin Skywalker were two completely different characters? Lucas didn't make the decision to combine the two characters until it was suggested by the screen writers he had help him write the script for ep V, Leigh Brackett and Lawrence Kasdan. You can almost see it in the dialog between Obi-Wan & Vader before they went to blades in ep IV:
Vader: When I left you I was but the learner now I am the master.
Obi-Wan: Only a master of evil Darth.
Obi-Wan calls him "Darth" as if it were his first name ... because at that point in the story development it WAS his first name not the first name of every Sith Lord in the history of the galaxy ... heck at that time we didn't even know what a Sith Lord was at that time.
That massive change had its greatest effect on the story of the prequels as in the original backstory for it Lucas had Obi-Wan & Anakin the same age, best friends, and fellow Jedi Knights fighting in the Clone Wars. Darth Vader was Obi-Wan's apprentice who turned to the dark side and joined the Emperor in destroying the Jedi Order and in the process he killed Anakin Skywalker. Combining the characters made much of that backstory "wrong".
Another "mistake" you are making ... "Which is obviously Leia" ... in 1980 is was NOT obviously Leia. In 1980 the vast majority of fans didn't believe that Vader was actually Luke's father. As for who the "other" might be many fans thought it might be Han ... of course we were all wrong also.
The decision to make Leia the "other Skywalker" was made for two reasons. First in the story Lucas got stumped with what he could have Vader say to Luke that would cause Luke to attack his father in a dark side fueled rage in their final duel in ep VI. To solve that he made Leia the "other Skywalker" and had Vader threaten her. The other little know reason was to convince Harrison Ford to reprise his role as Han in ep VI. Ford thought that it was the perfect end for the character to die in order to save his friends. Hamil & Fisher had signed 3 movie deals but Ford had only signed a 2 movie deal in the beginning. When Lucas showed Ford that Luke & Leia would be revealed to be siblings and that Leia & Han would end up together he signed on for the movie.
If you really want to get "deep" into it making Leia the "other Skywalker" actually eliminated the last trilogy of the saga because that was supposed to have been about "the other Skywalker". That's a pretty BIG effect on the overall story of the saga if you ask me. In the SW EU (Expanded Universe of books) Leia actually DOES become a Jedi, a very powerful one. The EU IS canon and IS part of the overall story of SW but you would be right in saying that it doesn't get shown in the movies.
The source for much of this can be found in "The Secret History Of Star Wars" written by Michael Kaminski and various entries in Wikipedia.
May The Force Be With You ...
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