Honestly if it’s bad writing in ROTS it’s bad writing in ROTJ as well. They’re both retcons. Leia was never adopted until Lucas decided to make her Luke’s sister.
Sure ROTJ doesn’t necessarily directly contradict anything in the previous films, but, and maybe it’s just me, there still seems to be a logical leap in Luke and Leia’s conversation. Leia already knew she was adopted (and Luke for that matter)? Seems to come out of nowhere and never fit right for me.
We never really see Leia properly react to the terrible truth that Vader is actually her father
Yeah that has always bothered me. She reacted more to finding out that Vader was Luke’s father than her finding out her own heritage. You would think that finding out the right hand of the Devil was your father would at least warrant a “Come again” or her jaw dropping slightly. Anything other than her pretty much brushing it off. I attribute this to George’s inability to work with actors and Carrie Fisher’s so-so acting.