Slice the tops off of the strawberries and slice them in half. Remove the leaves from 4 of the mint sprigs. In a pitcher, combine the sliced strawberries, mint leaves and sugar, and muddle with a muddler or wooden spoon until the strawberries are mashed. Add the raspberries and lightly mash but leaving them somewhat intact. Add the cranberry juice, lemonade and rum, and stir to combine. Add the seltzer and stir. Pour into 4 glasses filled with ice and garnish each with a sprig of mint.