Speaking of similar scenes in the same movie: the sister reveal is explained 3 times! First Luke get the news, then he breaks the news to Leia, and then Leia has to explain it to Han. It makes sense for the characters to get this information, but for the audience it’s the exact same reveal repeated 3 times. And it’s boring.
Also, Leia seemingly has no reaction to the fact that this implies Vader is also her father. (Luke tells her Vader is his father in the same scene.) Maybe her reaction is delayed or she doesn’t immediately make the connection. She’s upset a few minutes later after Luke leaves, when Han comes by. But it’s implied she’s upset mostly because Luke had to leave.
You’d think she’d have a much stronger reaction, like “Holy shit, you mean the evil cyborg that tortured me, and tortured Han, and stood by and watched as my whole planet exploded, etc. is my father??”
Of course, the real reason she doesn’t react is because Leia and Vader never interact again in the story, so the story doesn’t bring up Leia’s feelings about this since it’s not significant to the story and there would never be any real payoff.