First off, I would like to say that I do love the Star Wars saga (as a whole mind you ). There is a serious plot hole/ inconsistencies in the story. In episode IV A New Hope, Darth Vader confronts Princess Leia, we learn from episode VI that Leia is Luke's twin and the daughter of Anakin Skywalker. Anakin who at this point is Darth Vader is completely oblivious to the fact that his children survived, however, the question is why didn't he sense the force in Leia. Later once she is on the death star in the brig he enters her cell once more to question her, yet he did not sense the force. Later that same episode when chasing Luke in the death star's trenches he senses the force within Luke from his tie fighter. He says "The force is strong with this one.". This lead to Vader frantically searching for Luke at the start of episode V.
In episode V, Vader is once again face to face with Leia on cloud city, after the deal struck by Lando Vader is awaiting solo and the rebels in what appears to be a dining area, once again he should have sensed the force in Leia, after all her being Luke's twin and his daughter would make her equally as strong in the force.
Once again in episode VI Luke, Han, Chewy, and Leia were on a shuttle to Endor, their mission, destroy the Death Star's shield generators, Vader sensed Luke's presence from a nearby star destroyer, yet he did not feel Leia. How can this be, especially at this point in the story when we are made aware of Leia being Luke's twin. Also why did Yoda and Obi-Wan not make any attempt to train Leia, if Luke had failed that would have been the end of the Jedi and the Sith, because the battle waging within Palpatine's chambers had no barring on the battle raging around the Death Star and the Death Star went down and the rebels won without any Jedi help, however, they Obi-Wan and Yoda would not have known that and they bet the farm on Luke and Luke alone.
Now back to the ever growing plot void, in episode I, we learn that no real Jedi training is needed for a trained Jedi Master or Sith Lord of Vader's caliber to sense the force in someone, untrained like say Leia, after all Qui-Gon felt the force in Anakin even though Anakin was not aware he had it, albeit it was Anakin the chosen one but once again Leia being his offspring should have given off a strong aura of the force. Thus in short he should have sensed Leia's power the moment they first met face to face, way in episode IV.