I doubt they would succeed in keeping it a secret from her and i imagine archer giving rin a smack if she tried.
Well, obviously they won't
succeed, because she already knows about it. But, at the same time, she doesn't want
them to know she knows about it (because it would imply she was a master), and they don't want
her to know. So, the charade is likely to continue reasonably well, I suspect.