I agree that there’s too much written to completely separate Luke and Leia, but they don’t have to be literally related by blood to be spiritual siblings. To take a page from TROS, they could be a Dyad in the Force, and it could even go back to the Sith’s ‘Rule of Two’.

In the Original Trilogy, the only proof that we have of Luke’s sister is from the ghost of Obi-wan, who has already lied to Luke about his dad and has just told him that his own words are only true from “a certain point of view.” No changes to Obi-wan’s dialogue are really necessary, he could just be lying about the sister thing because it’s easier to explain to Luke why Leia would be suddenly gaining strength in the Force than to try and bring up Force Dyads or the Rule of Two.

At this point the only problem with this is two scenes from the end of ROTJ, and I’m perfectly happy ignoring that to support this headcanon 😉