Yeah, I agree with @Skribulous here, if they wouldn't recognized Reimu as a yukkuri, they would have kicked her out the herd or even killed her on the spot. I think that they know that that thing is Reimu, their child/sister, but that thing is so annoyed that this is a very good opportunity to "teach" her a lesson or something like that without getting unexpected attention or uneasiness.
Edit: as you can see @ukshadow, Reimu still call her "this kid", that's the strongest evidence that they can recognize her.