Horror B-Movie, Grandmother’s Spell Book Protects her Grandkids

My recollections of this film are vague but it was a horror B-movie whereby people are being chased by the undead. Based – at least toward the end of the story – around some teenagers who had found their grandparents gone/killed but discover their grandmother was a good witch. They find her spell book and find out it protects them from the undead, even able to protect the whole house they’re in by shooting magical beams at incoming undead (or was it a magic barrier? Not 100%). By the end of the film they make their way to either a house or gas station only to find their grandmother has become one of the undead and uses her own spell book against them. I vividly remember her suddenly appearing and saying, “Surprise!” before she blasts them. Set in, I think, southern USA.