Well, we still don't know the exact terms and details of the Gambit... we know that Kane is to be tested and deemed worthy (or not) and it's very likely that they're all bound to secrecy about him until he reaches a certain level (probably K10)... it's also quite likely that IF Kane were to somehow get that info before time, he'd be disqualified because his testing could be considered "tainted" or whatever, which is why no one is telling him jackshit, not the sisters, not his mom, not Yvette or Elena. They're all probably bound to secrecy about the whole thing under serious threats.
Also, part of the secrecy is that he is also supposed to breed a sister so that Alexander gets his desired descendant, so if he were told that, it could be done without following the terms of the Gambit... in fact, Dominique tries to cheat. But anyways, as long as we don't know the full deal of the Gambit, it's kinda hard to explain everything... but so far it mostly makes sense.
The DeKocks are a bit more puzzling, they obviously know what's going on but are also keeping quiet... but I'm sure we can come up with some reasonable speculation as to why they're doing it... it's probably part of a powerplay anyways, but both groups are super secretive and don't like showing their hand... there's obviously a lot of scheming going on, I'm sure we'll get more info soon enough.