The grandmother/aunt/old lady was part of the cult or was disposed of by the cult.
The cult happens to show up after she discovers the boy had found the map, and is never seen again. Maybe she told the cult. Maybe the cult got her by telling her she had to work the night shift.
Consider the following:
- Troy: whiny orphan raised by overbearing relative who doesn't want to talk about the past (Luke Skywalker).
- Zapp: Gruff, cynical, pragmatic former bad guy with a temperamental old vehicle, allegedly cooler than Troy (Han Solo).
- Mike Pipper: Grizzled old hermit who was a friend of Troy's father, knows ancient mystical secrets, and occasionally smokes a pipe (Obi-Wan Kenobi).
- Satoris: Dresses in black, speaks with an unnaturally deep voice, possesses vaguely-defined magical powers, leads an army of faceless ineffectual mooks (Darth Vader).