Luke and Leia weren't meant to be siblings - that was forced into the script. I believe Gary Kurtz gave an interview last year explaining some of the reasons he left during pre-production:

http://herocomplex.latimes.com/2010/08/12/star-wars-was-born-a-long-time-ago-but-not-all-that-far-far-away-in-1972-filmmakers-george-lucas-and-gary-kurtz-wer/

Here's another one where Kurtz details the original nine episode arc:

http://www.theforce.net/latestnews/story/gary_kurtz_reveals_original_plans_for_episodes_19_80270.asp

Not turning this into a bash Lucas thread. While ROTJ has faults and the criticism in the OP is valid, it's still a very good movie worthy of the OT and vastly superior to the awful prequels.