UrsusArctos: For Seele's entire abominable interrogation to succeed, it required (1) Ritsuko not to know that Gendo sent her, (2) Ritsuko to be exceptionally resentful of Rei and (3) Ritsuko to believe that Kiel was telling the truth. More in the additional comments.
Additional Commentary
UrsusArctos: The first part would be achieved if Ritsuko were summoned by Section Two agents in the same manner that they visited Misato in Episode 21. Given that Section Two is demonstrably rife with Seele agents, it makes sense that she was summoned without being told of the fact that Gendo had let Seele interrogate her.
The second part seems to be because Kiel lucked out. Gendo himself doesn't know that Ritsuko is resentful of Rei, and it's clear that Seele don't know much about Rei herself or this would be a very different conversation. The relationship between Gendo and Ritsuko would be far easier to put together, and it makes sense that they'd try to drive a wedge in that particular relationship to teach Gendo a lesson.
What beats me is the last part. If Ritsuko knew that Seele knew about their relationship - "I can see why Ikari would want you by his side" is as close to "I know of your relationship with Ikari" as is possible for a Seele member - how did she never think of the possibility that they were lying, trying to attack her mentally the way they're implied to have done so physically? It's because Ritsuko has convinced herself that Rei is being groomed as a substitute for her, and Kiel exposed Ritsuko to one of her worst possible fears at a moment of deep vulnerability. She doesn't even stop to think that it might be a cunning lie. While it's possible that Kiel has a strong perverse intuition about these things (and has convinced himself that Gendo is protecting Rei out of perverted lust rather than entirely different reasons), it's perfectly possible that he was subjecting Ritsuko to a prolonged mental attack on multiple fronts and simply got lucky.
