Put 2 cups raspberries in fine-mesh strainer placed over large glass measure or pitcher. Pour boiling water over raspberries. With back of spoon, press berries to release juice. Discard berry pulp; reserve juice (should have about 1 cup). Chill.
In a blender container combine cranberry juice and reserved raspberry juice. With blender running, add ice through opening in lid. Blend until slushy.
Transfer juice to pitcher. Slowly pour in carbonated beverage; stir. Serve over ice. If desired, add fresh raspberries and a lemon slice.