As far as I'm aware, the following are correct:
Kylie doesn't know that David knows (she might have her suspicions after the DNA test, but she's not sure);
David doesn't know that Kylie knows about the DNA test;
Kylie and Gail both know that the other one knows, as they have discussed it between themselves;
Nick is playing ignorant to Kylie and reckoning on that only he and Kylie know.
If you wasn't confused before, you should be now :)