I think there is a contradiction in Empire Yoda knows that Leia is in danger and tells Luke to complete his training and sacrifice her if necessary then when Luke leaves Yoda says “There is Another” i doubt that the original intention is that Yoda was referring to Leia because Luke was going to rescue her we all know that originally Luke sister was not going to be Leia and she was planned to appear in a future ROTJ sequel or maybe in ROTJ i think in the end they decided to make Leia Luke sister to avoid further complications or for marketing reasons but you cant deny that Leia being Luke sister was a last minute Retcon and it was forced

There were also rumors that that the other one was Boba Fett but i doubt it because he worked for the Empire so it is most likely that Yoda was referring to Luke real sister