You Guys are Getting Paid

One explanation I’ve heard is that in all the times he met with her in A New Hope, he was just trying to figure out what she had done with the Death Star plans and the location of the rebel base. He knew all along, but he never needed to tell her because it didn’t benefit him.

When Vader met Luke in Empire Strikes Back, it was a very personal and emotional battle. Vader was testing Luke and toying around with him, showing him how much he has to learn in the ways of the force. And only at the end when he won the battle did he reveal he was his father. He wanted Luke to join him in the Dark Side.

Vader didn't know that Luke and Leia were his offspring because after he was beaten by the high ground his ability to sense through the force was severely diminished. He also wasn't looking for them because he thought they died along with Padme (rip). Vader only learned that Luke was his son after Boba Fett was tasked on hunting him down (Vader comics) and told Vader that Luke's name was Skywalker. Vader flipped out and almost tore the command deck apart, then he went to the Emperor who confirmed it, Luke was his son. Vader/Anakin never knew there were twins, so Vader thought that he had found the only offspring of Anakin Skywalker. When he was searching Luke's feeling on Death Star 2.0, he realised that he and the Emperor had been wrong all along, and that there was "a sister"

I heard it was because Leia didn't realize she had the force meaning Vader couldn't sense it