M...I think he might be expecting something from her, perhaps that can be why he is letting her getting away so easily and has let her to be close to Yuuki. What? I do not know, but I can not find another reason why to allow all of these...there is a strange remark that Sara makes on a previous chapter about Kaname...
So Sara with all these has not yet touched Kaname's red button that he would go
and exterminate her? its hard to believe.
There is Yuuki but also her tablets are now very dangerous, so I think that Kaname is waiting for her to say that secret and create even more reactions there...
I also had the same impression with the last chapter when he had send to Cross that message for Yuuki like saying her to trust Sara, the moment that Sara was asking for trust in order to share that secret, perhaps Kaname wants more conflict to arise but you can never know.