How would she have heard? She tried to stop Chris (pre-utterance of Grandma) and got hit by the car and was knocked unconscious. The granddaughter couldn't just utter the word a mile away outside of the car and then "poof" the grandmother takes over.
Similarly, Walter attacks Chris before she says "Kill him, Grandpa!" It's the flash that switches him from Grandpa to Walter.

It was all explained outright in the scene in the TV room. And it was shown how the flash was what screwed with the operation; got the small part of the original brain functioning again. Kidnapped dude screamed GET OUT! because of the flash.