i don't think you need to take too much responsibility at all to be honest

you've said no, unless otherwise notified
you've attempted to make contact when it appeared in the paper
you've spoken to the sister who she travelled with - if they are close enough to car pool, they are close enough to say "hey, it's only my older kid playing, not the younger one"

to be honest, i would think that she DID know, but decided to go along to watch her niece play - and is now going to make waves just because she's "one of those people"

apologise for not being able to reach her, but don't accept responsibility - remind her that it was always a no go unless otherwise advised - and leave it at that.