Pour gelatin into a large mixing bowl. Add 3 cups boiling water and stir until gelatin has completely dissolved, about 2 minutes. Refrigerate mixture until gelatin just starts to set, about 90 minutes.
Remove gelatin mixture from refrigerator and whisk to loosen. Add sour cream and cranberry sauce to gelatin mixture and whisk until thoroughly combined. Pour into a mold or serving bowl and refrigerate until fully set, at least 1 hour.