I can't find that last quote at all, its not in the link above it -- just that Kurtz said for Jedi having Leia be the sister was the most eloquent way of tying it all up. And of course that's true.
I don't believe Leigh's original draft -- not sure how many there were -- was ever released back in 1980, so I don't recall having any of that info (I only scanned it obviously, I'll have to read it). But again, I'm speaking from 1980's reveals on... in 1978 there were a lot of crazy ideas afloat I'll admit - look at Splinter in the Mind's Eye.
My point was that at some point in the final stages of ESB, the idea of Leia as Luke's sister was present and being considered -- its necessity came when Lucas decided to end the whole thing and only to be revealed for the final chapter. I'm not sure Lucas had OK'd yet -- probably not as he always kept himself open then. But when Yoda says "there is another" I do not believe that it was agreed upon that it meant Luke had a sister across the galaxy or that it absolutely was NOT Leia. Lucas already had a major reveal for 1980 -- that was the focus, but they knew that the triangle had to be settled, that Luke couldn't hook up with Leia after she chose Han, and that introducing a "new sister" character would somewhat sideline Leia. Then of course there's that scene in ESB -- Williams musical cue with Leia "hearing" Luke that makes such a connection point -- obvious it can be read many ways and was likely intended to -- but I do absolutely believe it was read as Luke's connection to his sister and a prime example looking back of that cue. Perhaps in 1980 it had not be wholly decided but it was a serious option... the obvious option... and certainly became the truth.
As explanation, I devoured everything I could on Star Wars 1977-1983. Admittedly, I never got into the EU and never really absorbed SW again like I had in those early days. But that time period, that little 6 year window, I truly read everything I could get my hands on and had the kind of knowledge that many of you have now that I no longer possess. Also, my friend's sister never could tell me what was going on at LFL obviously, but on occasion she did play a game with me called "guess and see" and I would outline my theories based on what I was reading and assumptions I would make based on the films, and she would give me clues... sometimes I could tell I hit the nail on the head because she'd have to get up and end the game with a smile. It was once such occasion very early on but after ESB where I called the Leia sister bit and got that response. In 1983, she could finally confirm that that was the general decision for Leia very early on (though when that ultimate final call was made I do not know).