Quote Originally Posted by Uncle Big Hands View Post
I wouldn't expect her to study all of her history, but at least a little bit of it. Knowing he has a sister likely to show up is ever so slightly important.
Like I said earlier, she's an idiot.

And being stressed over the wedding is hardly an excuse to fail to notice the worlds most obvious fake.
The ponies are too focused on the wedding to notice anything different. Except for Twilight, of course.

And for your Idea Cadence would had to have left since the raising of the force field.

And she would have to have left alone, during a time Canterlot was considered under threat.

Which would make her and Canterlot guards massive idiots. Your theory requires everyone to be massive idiots.
It's still possible, and yes the guards would have been major idiots. Remember how that one guard allowed Twilight inside the library when she was trying to sneak inside during "It's About Time?"