Before the twins' visit
Granny Sweetkin is one of Stan's tourist trap competitors who prank the Mystery Shack every year. The year before Dipper and Mabel came to stay with Stan, she and the others duct taped Soos to the ceiling, leaving him there for 78 hours.
Stan also mentions Granny Sweetkin lit his car on fire "on two non-consecutive occasions." 
In "Roadside Attraction," the first stop of Stan's trip of revenge is Granny Sweetkin's tourist trap. While the gang is there, they tie one end of the yarnball to their RV's bumper, so that they unravel the ball as they leave. Granny Sweetkin turns out to be inside the yarnball, and when it unravels, she gets angry, runs after the RV and throws a giant knitting needle at it like a javelin, which impales itself into the back of the vehicle. She later vandalizes the Mystery Shack along with the other tourist trap owners.
Despite the cute decoration of her tourist trap, Granny Sweetkin is far from a sweet old lady. She can get very aggresive when provoked, swearing in an eldery manner and vandalizing others' property in vengeance. She can throw the giant knitting needles in her giant ball of yarn like javelines. Stan refers to her as "the most blackhearted proprietor in all of Oregon." 
Granny Sweetkin has light wrinkly skin and a long nose which is darker in color than the rest of her face. She wears a light blue dress with white collar and cuffs, and brown shoes. Her hair is gray and worn in a bun, held back by pins. Her arms sport numerous liver spots.
- ↑ 1.0 1.1 1.2 "Roadside Attraction"Jeff Rowe, Josh Weinstein, Alex Hirsch (writers) & Sunil Hall (director). Gravity Falls. Disney Channel. September 21, 2015. No.16, season 2.