Channel72 said:
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.

When Leia makes her aghast “Your father!” I always imagine her thinking, “Boy am I glad Darth Vader isn’t MY father! I can’t even imagine…!”