Sister brand of Slush Puppie

ICEE

This frozen treat is often found in convenience stores and shares its name with a chilly suffix you might add to "snow."

The clue refers to ICEE, a frozen drink brand that’s often found in convenience stores and movie theaters. It’s called the "sister brand" of Slush Puppie because both are popular slushie-style drinks, though they’re owned by different companies—ICEE is tied to the ICEE Company, while Slush Puppie is its own separate brand. The hint plays on their similarity as competing frozen beverages, making ICEE the straightforward answer.