Force powers are vague psychic/mystic stuff. We see Vader sense Kenobi, and he can tell that Luke is strong with the force when he is focusing on him. And there’s other stuff like sensing Alderaan blowing up or things happening in other places and times (aka literally anything the plot wants the character to see). But the rules just really aren’t clear, especially in the original trilogy. There’s no indication given that Vader should be able to sense that someone is related to him, nor would he be able to pull it from Leia’s mind since she doesn’t know either. If anything, he should have sensed the millennium falcon coming in to cover Luke during the trench run, that seems like the more obvious thing to be able to detect.
Troi has empathy, which is a much more clearly defined concept. And while they stray from that sometimes by letting her sense danger or other nebulous generalities, we do know that she should be able to sense emotions, intentions and honesty vs deception. That makes it stand out when the script seemingly forgets that she should be able to sense lies, deception, and malicious intent. Conman pretending to be a researcher from the future lies in order to steal from them and she can’t tell that he’s a fraud. The staff at a party are getting ready to grab weapons and take them all hostage, and she doesn’t notice. These are situations that call for things that are well within her established skillset.
Who says he didn’t? We only know he sensed that Luke was strong in the force during the death star trench run because he said it out loud. Presumably he has encountered many people with relatively strong force potential, but that’s probably something he’s going to keep to himself unless he has a reason to act on it. She’s a prisoner that he’s unlikely to recruit, and it’s not like she’s had any training. Hell, he probably dismisses people like her as being too old to start training anyway. (And if we bring in stuff from outside the movies where they explain that force sensitive people often become leaders, or larger than life figures that shape the course of events, then one would expect a lot of those people to run into Vader eventually.)
It may also be that force ability is more apparent once someone actually starts using the force. That would be logical and consistent with the fact that Luke is only really noticed by Vader once Obi Wan started telling him to use the force, and the fact that Vader and Luke could sense each other in space over Endor, but apparently that didn’t apply to Leia.
And again, he wasn’t trying to reach Luke on the radio to make him an offer once he noticed the force was strong with him, he was trying to kill him. The empire wasn’t determined to capture Luke until the emperor was able to sense him making waves in the force, when he was at the point where he was already using a lightsaber and capable of telekinesis, and when he had been identified as Vader’s son. At that point he had demonstrated actual ability, not just wasted potential, and the familial connection offered a plausible way to work around his opposition to the empire.
Force powers are vague psychic/mystic stuff. We see Vader sense Kenobi, and he can tell that Luke is strong with the force when he is focusing on him. And there’s other stuff like sensing Alderaan blowing up or things happening in other places and times (aka literally anything the plot wants the character to see). But the rules just really aren’t clear, especially in the original trilogy. There’s no indication given that Vader should be able to sense that someone is related to him, nor would he be able to pull it from Leia’s mind since she doesn’t know either. If anything, he should have sensed the millennium falcon coming in to cover Luke during the trench run, that seems like the more obvious thing to be able to detect.
Troi has empathy, which is a much more clearly defined concept. And while they stray from that sometimes by letting her sense danger or other nebulous generalities, we do know that she should be able to sense emotions, intentions and honesty vs deception. That makes it stand out when the script seemingly forgets that she should be able to sense lies, deception, and malicious intent. Conman pretending to be a researcher from the future lies in order to steal from them and she can’t tell that he’s a fraud. The staff at a party are getting ready to grab weapons and take them all hostage, and she doesn’t notice. These are situations that call for things that are well within her established skillset.
Maybe not if they are related, since neither of them knew.
But shouldn’t Vader detect the force in someone? Like Qui-Gon Jinn in the prequels.
We now know both his children are strong with the force
Who says he didn’t? We only know he sensed that Luke was strong in the force during the death star trench run because he said it out loud. Presumably he has encountered many people with relatively strong force potential, but that’s probably something he’s going to keep to himself unless he has a reason to act on it. She’s a prisoner that he’s unlikely to recruit, and it’s not like she’s had any training. Hell, he probably dismisses people like her as being too old to start training anyway. (And if we bring in stuff from outside the movies where they explain that force sensitive people often become leaders, or larger than life figures that shape the course of events, then one would expect a lot of those people to run into Vader eventually.)
It may also be that force ability is more apparent once someone actually starts using the force. That would be logical and consistent with the fact that Luke is only really noticed by Vader once Obi Wan started telling him to use the force, and the fact that Vader and Luke could sense each other in space over Endor, but apparently that didn’t apply to Leia.
And again, he wasn’t trying to reach Luke on the radio to make him an offer once he noticed the force was strong with him, he was trying to kill him. The empire wasn’t determined to capture Luke until the emperor was able to sense him making waves in the force, when he was at the point where he was already using a lightsaber and capable of telekinesis, and when he had been identified as Vader’s son. At that point he had demonstrated actual ability, not just wasted potential, and the familial connection offered a plausible way to work around his opposition to the empire.