A Rational Reason to Hate "A Canterlot Wedding"

http://mythcreants.com/blog/six-plot-excuses-no-one-wants-to-hear/

See the first one.

To explicate, since, if anyone bothers to read these things, I know they'll go after the angle of Cadance being Celestia's niece, Twilight has been the Faithful Student for roughly the same time. As a young teen, Cadance is an Alicorn, and looking after Twilight. Around the same time, since Twilight's size when being foalsat by Cadance is quite similar to when she got her cutie mark. If anything, she looks a bit bigger with Cadance. There is no way Celestia didn't know they'd know each other.

If Celestia knows that Cadance and Twilight knew each other well, then she trusts (or should trust, if she's in character) Twilight's instincts, particularly friendship related ones. Twilight, if she's being rational, ought to be trusted as the faithful student, babysat by the niece. Celestia, if she's being anything remotely resembling watchful, ought to pick up on discrepancies in "Cadance's" act.

Celestia ought to be watchful. There's been a threat made on Canterlot, and her niece is still insisting on getting married. She should be concerned for the ponies she cares about, and, with the exception of Luna, Cadance and Twilight are it. Disregarding that, she should have concern for Shining, because he's the one protecting the city.

The story might have been better if Chrysalis gets caught, and then they search for Cadance, only for Chrysalis to throw in a last monkey wrench. So, it wasn't even necessary for things to go the way they did.