Okay. I don't know why I'm investing in this so much, but lets play this out:
Random middle-schooler who's not a control but is trying out the sharing and hasn't been offered full membership yet (RK): Hey Tom.
Tom: Hey RK what up?
RK: Did you hear about that teacher at my school that went nuts today, and Chapman took us out of class
Tom: Yeah, crazy huh. (thinks to self: idiot chapman, letting that get out of hand. But with the pool and the shuttle out of commission, we couldn't take the whole room anyway I suppose)
RK: Yeah, oh hey, I meet your cousin phil
Tom: What?
RK: Yeah, well I didn't really meet him. In that class with crazy teacher, Jake brought in a new kid, told the teach it was his cousin Phil from out of town
Tom: Oh, okay (misc small talk continues, Tom thinks to self: weird the host has no cousin Phil. And from out of town. Can't risk word getting out of suspicious activity hear.)
Later
Tom:(I have no idea who this Phil, and no one else seems to have noticed him, what with the "crazy" teacher. I'll have to talk to the human)
Hey Jake, RK says you brought in a cousin Phil to school the other day. Who's that
Jake: Oh, um, well, it's no my, our cousin, duh. Actually, he was uh, Marco's cousin
Tom: Marco's. Then why were you saying he's your cousin
Jake: Oh, well, the kids a little weird, Marco was kind of embarrassed to be related to him. And I owed him for letting me borrow some notes, so I agreed to pretend he was my relative.
Tom: Weirder than Marco?
Jake: Crazy right. But yeah.
Tom: Weird. You and friends are stupid (generic small talk continues. Thinking to self: Well, the boy and his friends are stupid, this isn't too weird. Besides I have more important things to deal with. The old yeerk of my host decided the kid is not threat, and he's probably right. Plus the Visser is hatching a new plan to.......)
Scene
Now I was a bit sardonic in that scenario, but do you not see how an arrogant master of the galaxy Yeerk would was over look that tidbit of information that almost certainly would not have been pointed out to him while dealing with plans to take over the world. The yeerk had a whole live apart from Jake. He would have been suspicious of everything, but he has not reason in to cue in on a little brother being weird above anything else in the world