This… Isn’t the answer.
More to the point, every answer to this question is talking about something completely different. Vader was distracted. Vader was only clued into Leia’s existence after Luke thought about her. Vader had too much on his plate…!
But this answer, even among all the above, is reaching at things that just make no sense. By your logic, every force user would be crippled at the battle of Geonosis, with hundreds of jedi and sith present.
Qui-Gon and a young Obi-Wan took on Darth Maul, where the latter was able to use his ability without seemingly any hindrance at all. Similarly with Obi-Wan and Anakin against Count Dooku. When Yoda steps in, it’s 3 jedi against Dooku. There’s no evidence of Dooku bring weakened, as he matches Yoda in force abilities without much effort. By your logic, the sith in both encounters should have been weakened. That is clearly not the case.
The simple and obvious truth is this: at the time of writing, Lucas never intended for Leia to be Luke’s sister (hence their kiss) and so, there simply wasn’t anything for Darth Vader to sense. Retconning that and then adding in some wishy-washy explanation just trashes the credibility of the franchise.
Leliana’s French-ish accent is all I remember from that game. It’s a shame she was almost nonexistent in subsequent games. Mass effect fully brought back recurring romanceable characters so you could continue role playing.
Origins remains my favourite Dragon age game to date. I really hope the next one is a return to form!