Summary. Darth Vader's failure to recognize Leia as his daughter in Star Wars: A New Hope stems from George Lucas's late decision to make Luke and Leia siblings. Leia's Force abilities and her lack of awareness of her true identity also contributed to Vader's inability to sense their familial connection.
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.
Darth Vader didn't learn he had a daughter until Return of the Jedi, when he stood before Emperor Palpatine. The Emperor's power probed Luke's mind, attempting to goad him to fall to the dark side, and he discovered Leia's existence. "Sister," the Emperor taunted.
She finds it easy to accept Luke as her brother, but she is horrified by the idea Darth Vader was her father. Leia couldn't even understand why Luke accepted it so easily, and somehow believed there was good in their father.
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.
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.
In Bloodline, Leia does sort of come to terms with Anakin/Vader. It is not forgiveness, really (this is the man who watched her planet get blown up, tortured her, tortured her husband, and was a known ruthless killer of dozens if not hundreds of people).
No, Anakin was never made aware that he had children until he is initially confronted with Luke, and then suspects between ANH and ESB that Luke is his son. Obi-Wan comments that "the Emporer knew, as I did, that if Anakin were to have any offspring they would be a threat to him.
Luke and Leia never sensed that they were twins until Return of the Jedi despite both being sensitive to the Force. Anakin Skywalker's twin children are the original Star Wars trilogy's driving force, having been separated after Padme's death.
Marvel's Poe Dameron comic series reveals that Leia actually went to Naboo to learn more about Padmé, trying to get to know her mother through her friends and confidants. No doubt she visited Padmé's tomb herself.
Yes. In Truce at Bakura, Anakin's ghost appeared to Leia, begging for her forgiveness. She rejected him. Only by Tatooine Ghost did she start to come around to the father underneath the suit, just in time for her to finally have kids in the Thrawn Trilogy.
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.
During the battle, Anakin, who had been known as the Sith Lord Darth Vader, was redeemed by Luke and brought balance to the Force. However, the redemption cost Anakin his life, having been mortally wounded by the Emperor, Darth Sidious, while killing his former Master.
No. Neither Luke nor Leia found out they were siblings until the third film, Return of the Jedi. Indeed, it wasn't decided that they were siblings until the making of that film (because Lucas no longer wanted to make further sequels about a search for Luke's sister, and decided to wrap everything up with Jedi).
Yes. In the book 'Tatooine Ghost', Leia learns of Anakin's life prior to his evolution into the Darth Vader personae. This revelation leads her to forgive him. She even named her third child in memory of her father.
Bail Organa and Obi-Wan fabricated Padme's death and the twins' story to keep Luke and Leia hidden after the events of Revenge of the Sith. By maintaining the secret, the twins remained safe, eventually allowing them to orchestrate the Emperor's downfall and the Rebellion's victory.
Within Vader still resides a part of his former self, Anakin Skywalker, and it could be argued that this is why he can sense Luke while the Emperor cannot.
