Unless it was subject to some kind of stress (being in a car all night, with the temperature way below zero, then being brought immediately into a 75 degree house, or having been sat on a little while ago) it's probably either defective or it got scratched exactly on a fracture plane by a tiny carbide particle (they float in the air, which is why you never wipe a screen with a dry tissue unless it's got a screen protector on it).
The problem is that it's not a common Tab 3 failure (it's so rare that this is the first one I've heard of), so you might have a problem convincing the store or Samsung that it wasn't something your brother did that broke it.