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.
Darth Vader only learned he had a daughter in Return of the Jedi - but there's no evidence he realized she was Leia before his death and redemption. Darth Vader may have never known Princess Leia was his daughter in Star Wars and never cultivated a relationship with her, even when he was a Force ghost.
In Return of the Jedi (1983), Leia helps in the operation to rescue Han from the crime lord Jabba the Hutt and is revealed to be Vader's daughter and the twin sister of Luke Skywalker.
He knew Luke was, but not Leia at first. In the first two Star Wars films, Vader comes face to face with her multiple times and doesn't recognize her as his daughter. As for Luke, he knew from the moment he heard the name he had a son and tried to find him.
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.
Did Vader Know Leia was his Daughter After Sensing Luke Had a Sister in Episode 6? (Legends)
Did Vader sense Leia was his daughter?
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.
In the post-2014 canon universe, the story that reveals Darth Vader discovering Luke Skywalker's identity (and thus the fact that the Death Star's destroyer is his son) is issue 6 of 2015's Star Wars comics by Marvel, which takes place sometime shortly after A New Hope.
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.
Similarly, her rebellious, rule-breaking and trouble-making nature resembled Anakin more than Padmé. However, both parents were adventurous and stubborn people. Thus far, her leadership skills are the one thing missing from Leia's portrayal in Obi-Wan Kenobi. It's well justified since Leia is still a child.
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.
It wasn't until Bloodline that Leia began to truly understand Anakin Skywalker, in a situation where she too felt tempted. "She'd always wondered what had led her father to turn to the dark side," Leia reflected, "to become Darth Vader. She'd imagined it came from ambition, greed, or some other venal weakness.
In the Star Wars canon, there is no indication that Leia ever spoke with Anakin's Force ghost after discovering he was her father. Anakin's Force ghost appeared to Luke Skywalker at the end of "Return of the Jedi" but did not appear to Leia in any of the subsequent movies, books, or other media.
Ever since Return of the Jedi, Star Wars has reframed Luke and Leia's kiss as lacking real romantic intent - something Leia did solely to get a rise out of her real love interest, Han Solo.
Vader was hunting down the pilot who blew up the Death Star, and hired Boba Fett to find them. Boba was unable to, but he did give Vader a name: “Skywalker.” This, combined with his knowing Luke was Force-sensitive and had been trained by Obi-Wan, was what made him realize the truth.
Rather, it is still the case that the one and only Chosen One is Anakin Skywalker, a being who was conceived by the will of the Force itself, and who had no father. But Rey does share the mission of the Chosen One, as indicated by Anakin's words on Exegol.
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.
Returning to Dagobah to complete his training, an ailing Yoda told Luke the terrible truth that Vader was his father, Anakin Skywalker. Rather than destroy his father, Luke sought to bring him back into the light. At 900 years old, Yoda passed away, telling Luke that there was another Skywalker.
The two recognize Luke's similarities to Anakin and try to prevent him from sharing his father's fate, but they let him remain Luke Skywalker instead of having him go by Luke Lars out of respect for his grandmother, Shmi Skywalker.
Why didn't Darth Vader search for his lost son Luke? He was under the impression that Padme died from his actions, and their child (he didn't know she was pregnant with twins) also perished. He didn't realize Luke was alive until after the Battle of Yavin, at which point he did begin searching for him.
And during the conversation, Padme is holding her stomach and appears to have a bump, seemingly confirming she was pregnant long before Revenge of the Sith. However, she chose not to tell Anakin, presumably because he was about to go into a battle.
Besides the obvious out-of-universe explanation that Vader needed to be unaware of the fate of the children (and their number), how is it possible, with all the technology depicting in the Star Wars series, and with Anakin's Force sensitivity, that Padme (and therefore Anakin) or Anakin (and therfore Padme) not be ...
