Alderaan's princess knew she had been adopted by Bail and Breha Organa after the Clone Wars, but she had no idea who her parents were. That was until Luke Skywalker told her the truth - that he was the son of Darth Vader, and that she was his sister.
Claudia Gray's novel Bloodline established that Leia struggled with the knowledge that Darth Vader was her father, as her memories of the Dark Lord of the Sith were the stuff of nightmares.
How did Leia react when she found out Vader was her father?
Luke, Leia, and Han are on Endor. First, Luke tells her Vader is his father. Her first reaction is one of slight revulsion, perhaps even of him. She gets a very concerned look on her face.
There are clues in Star Wars to suggest that Luke and Leia did, in fact, know vaguely about their relation to each other much earlier than Return of the Jedi, but perhaps, were not yet Force-sensitive enough to have the full picture.
In canon, Vader found out because Luke told Boba Fett that he was “Luke Skywalker.” This was explained in the canon comic Star Wars (2015) 5. Vader only sensed Leia was Luke's sister because Luke was thinking about her on the Death Star II. When Vader tortu…
Leia's Force abilities and her lack of awareness of her true identity also contributed to Vader's inability to sense their familial connection. Vader's denial of Anakin Skywalker's legacy and his fear of getting hurt again prevented him from realizing the truth about his children until later in the original trilogy.
Yoda. Yoda's knowledge of Luke Skywalker's true parentage was also confirmed in Return of the Jedi. As with Obi-Wan, Yoda believed that Anakin's fall to the dark side made him unsalvageable, with the Darth Vader persona being his only identity following his ascension to Sith Lordship.
Leia was not using the Force or was currently Force-sensitive during her confrontation with Vader. She began training as a Jedi years later. All these reasons prove my point: Vader didn't know Leia was his daughter, and felt no presence of the Force in her, as she was not currently strong with the Force.
In Star Wars Legends Leia was visited by the force ghost of her father Anakin Skywalker, the man who used to be Darth Vader. It wasn't a pleasant meeting as Leia was unable to forgive him for the things he'd done no matter how hard he begged, and it was the only time she saw him.
He eventually became so desperate to find a way to prevent Padmé from dying that his fear was ultimately the main reason that led to his fall to the dark side. Vader screaming for the loss of Padmé. Even as a Sith Lord, Anakin still loved Padmé very much, but felt great guilt for his actions against her.
Darth Vader acts like he doesn't even know C-3PO or R2D2. They were with him since he was a child. He did everything he could to forget his past. Also, his plating had been replaced many times, and protocol droids all look very similar.
C3PO was clearly created from some sort of kit or existing prototype since every protocol droid aside from coloring looks the same. He didn't recognize him because again, they all look the same & it was decades until he saw him again. There was no reason to believe he or R2 even existed anymore.
While their affection had to remain hidden, the two kept in frequent contact, even throughout the Clone Wars. But despite this, Anakin found out that Padme was pregnant only a couple of weeks before the twins were born. So, why did it take so long for her to tell him?
The real reason why Obi-Wan Kenobi, the Lars family, or Darth Vader didn't act in the original Star Wars trilogy as though they knew C-3PO or R2-D2 previously is that the prequel trilogy, in which they first meet the two droids, had not been written yet.
Dreams and Visions: Leia might have experienced dreams or visions throughout her life that provided her with glimpses of her mother. These dreams could have been a result of her Force sensitivity and her connection to Padmé's residual energy in the Force.
R2-D2 was present when Padmé gave birth to Leia and Luke, so he knew Luke was Anakin's son – but what he didn't know was that Anakin became Darth Vader. In addition to that, Anakin's fall was surely a painful event in R2-D2's life, one that he definitely wouldn't like to remember or even talk about.
