The writing of this scene is some of the best the game has to offer in my opinion.
A big part of Shadowhearts character is that she is as stubborn as a mule, and allergic to coercion. Throughout the game, if you try to force her to say/do something you will be met with either a sarcastic comment, or she will do the opposite out of pure spite. And in the Shadowfell this behavior is dialed up to 11 because it's such a stressful moment.
Like Ecc2ca has already said, you can see the cracks in her sharran persona during act 1&2. Even if she doesn't see it herself. And then Dame Aylin is able to shatter it for a moment with her speech. None of it is obvious of course, and even if you pick up on all of this there's still no hard guarantee as to what she'll do. But that's the beauty of this scene for me.
