I think this is the first time I’ve seen an idea like this suggested. One one hand it allows you to not make them technically siblings with the least amount of trimming to the film, but I can see how Luke still calling Leia his “sister”, even if it is meant as spiritual sister, could be a little confusing for a viewer.
I wonder if in ESB, Yoda could say, “No, there will be another” or, “No, there will be others.”
You could take it from his line, “You will be.” Plus, it would take the weight off WHO is the other, when it could be interpreted as Yoda saying that another hero will eventually rise to the occasion. He is 900 years old, after all, so this isn’t the first hero he has trained. I don’t think having Yoda say he will choose another is totally necessary, and this might be a little more straightforward. Because how would Yoda know that Luke would “choose” another before he potentially dies? I suppose it might help get the idea across, but we already know Luke and Leia have a special connection. And that is enough to explain why Obi-Wan would call them twins in the Force.
Really, the whole “there is another” line, and having to tie up that loose end by making Leia the other, all functions narratively as a sort of red herring. We just need to believe that Luke isn’t the last hope, and that if he dies or turns, the story could go on. So we are told there is another, and that Leia is that person, but since Luke succeeds we don’t really get a resolution to that potential back-up.
I remember doing an edit to ROTJ where Luke senses Leia getting shot, and that brings his feelings to the surface, allowing Vader to read his mind and discover the secret he’s hiding. If you wanted to play up there connection a bit more you could potentially do something like that. Maybe Vader could say, “Sister. So, you have a twin in the Force.”
I do see the strength of making their bond ambiguous, because the viewer could choose to interpret that as them being biological siblings or spiritual siblings. If the kids bothers you, then this would make it less weird since you can interpret this as them not being actually related.
I think if this was something I was going to change, I would just make Luke’s sister someone we never meet onscreen, like the original plan was. Definitely for a “what if Return of the Jedi was the end of the saga?” edit.