All along

Celestia thought she knew her sister like the back of her hoof. An incident with a changeling revealing spell was about to make her deeply question that belief.

Celestia thought she knew her sister. She was sure that nothing could surprise her anymore. She was certain that there was nothing more that Luna could possibly be hiding from her.
One attempt to identify a changeling presence in the royal guard later, she was coming to terms with how reality had decided to prove her wrong.