As always, my point of view, not necessarily share all of it with T4bbo.
1) One thing is to let your niece stay in your place for some time and a very different one is leaving her with the responsibility of closing the deal that will save your career/life. Specially if that means (and you know it does) she's going to need to be slutty towards the guy.
MC doesn't want to make a public slut of Christine, and at that stage, the moment Mr Silver touched her it would have been a game over, because there is no way she wouldn't have rejected the idea.
Sonya on the other hand is already a woman. She is married, she knows her options and somehow what she wants of her life. And you don't know her, just pay her, so she can be as slutty as she wants and you won't care (at least for now).
2) That's the point. First thing, Mr Silver talks to her in a moment you are busy, you don't even realize what he has done. Second, a game over in this update doesn't mean it will be in future updates, so it might progress from there. As I said in previous posts: patience, this is a game in development and need to think of it as what it is.