This is such a bad retcon, and ass pull on Clare’s part. I mean, I’m embarrassed for her at this point. Seriously, this is the first time we heard about Isabelle encountering a kelpie at Pandemonium, but all of sudden she mentions getting a party flyer from a kelpie at the club?
Although, admittedly there is a good possibility that Isabelle might have gotten to the club a lot earlier than Clary, and encountered the kelpie, but it’s still sloppy writing. Isabelle in the last ten chapters could have, at any time mentioned that she saw a kelpie at the club, and this little plot contrivance would have way more believable. However, at just at the right time Isabelle mentions the flyer.
So TL;DR: A good writer would have foreshadowed this way back in Chapter 1, but I guess good writing is too good to ask Clare for at this point. *disgusted*