Summary: He would never be her father.
Pairings: No pairings
Author's Note: I haven't read any of the books and apart from the two TV series I'm not all that familiar with the Star Wars EU, but I know from Wookieepedia that Leia wasn't terribly accepting of the idea of Darth Vader as her father, at least not at first (I don't know where characterization took her regarding Anakin after the initial revelation).
Disclaimer: I don't own Star Wars.
DNA and genes didn't make the parent—if it did then a good portion of the Galaxy was in big trouble, Leia included. It took far more than copulation and paternity to make a man a father.
And Vader would never be able to fill the shoes fatherhood required. He couldn't even fill the shoes humanity had left for him.
That Luke was her brother Leia accepted and even welcomed—he was a good man, a brave man, she loved him and was happy to call him her flesh and blood. But Vader? Vader she could never accept, only revile. Vader, who would always be a villain and a murderer in her eyes, irredeemable no matter what Luke said.
Leia could read the DNA results and accept that, yes, she was the biological—probably illegitimate—daughter of Darth Vader, formerly known as Anakin Skywalker, Jedi knight. But that didn't make him her father.
He would never be her father.
Fatherhood was so much more than blood. Fatherhood meant taking care of the children the man had and protecting them, teaching them, disciplining them when they needed it and always guiding them through life to make the right choices. Leia held a high standard for parenthood but didn't think she asked too much. Just what any decent man would do in regards to his children.
And Vader couldn't claim to have filled any of those obligations.
Instead, he had been one of the ones responsible for the death of Leia's real father, the man who had always loved and cared for her. And Leia couldn't help but wonder if Vader hadn't murdered her birth mother too—there was no word on who she had been except for the stories Sabé had been persuaded to tell her. She wouldn't put it, wouldn't put anything past him.
Vader was not a father.
He was the assassin of fatherhood.