I know that's the official Lucas story (and we know how reliable those are) but I do suspect he was giving it some thought even back then.

It's hard not to read ESB as being the film where the Leia as sister idea actually worked.

Perhaps it's because in ROTJ Mark is the only one of the big three still bothering so when ESB is re-watched with the sister reveal in mind Carrie's superior performance in ESB is more convincing.

The way Leia is sizing up Lando and then connects instantly with Luke's telepathic broadcasts does help sell the latent Jedi Leia idea. Maybe Lucas had Leia as a backdoor other should ESB be not as big a hit as anticipated or as it finally turned out he lost interest.

I've always found it easier to swallow that Leia is Vader's child than Luke, she does leave a trail of very sore throats behind her.