In a blender, put in the raspberries & sugar. Then, add in the 2 cups of lemon juice, 1 cup of water, and about 2 cups of ice. Blend well. Pour blender contents into a pitcher.
Then, add in the rest of the lemon juice and water to the pitcher.
Once ready to serve, add the liter of lemon-lime soda.
Notes
This recipe is intended for large gatherings - the amounts above yield about 1 gallon of the lemonade (so cut recipe in half for smaller quantities). If you want it extra slushy, then use frozen raspberries! You could also add the amount of ice. If you don’t want it slushy, just play with the ingredients, subbing water for the ice. Depending on the tartness of your lemons & the raspberries, you may need a bit more sugar. Once you’ve made the basic recipe above, sample & adjust as needed.