Yoda knows that Leia is in danger and tells Luke to complete his training and sacrifice her if necessary

Here’s the thing, though. Yoda doesn’t actually know if Leia’s in danger. The future is always in motion, and Luke’s vision of her and Han in danger could have easily been nothing more than a hypothetical vision of a possible future.

But yes, they definitely didn’t plan in any concrete way for Leia to be the other. Maybe it was an option they were considering at that point, but George hadn’t decided yet. I heard a rumor once that they were considering having Wedge turn out to be the other, but that always seemed like a stretch.