I believe that Vader might not have noticed that Leia was his daughter because she isn't "like him". She's more like her mother and I imagine that Vader repressed his memories of her quite a bit. As far as Luke Skywalker is concerned he has his piloting skills in common with Vader and is clearly "strong in the Force" despite his brief training. I can imagine that someone who blew up a Death Star would catch his attention. Besides the Force/deus ex machina can obviously detect things like that. He figures out that Luke has a sister easily enough, right?