Stir cream and sugar in a medium heavy-based pan over a low heat until sugar is dissolved. Remove from heat and stir in cranberry juice. Pour into a freezer container, cover and freeze for 1 hour or until crystals begin to form around the edge. Spoon mixture into a food processor and process until creamy, then transfer to a large bowl. Line a bar tin with baking paper.
Using an electric beater, beat eggwhites and extra sugar until soft peaks form. Fold eggwhite mixture through cranberry mixture. Spoon into prepared tin, cover with plastic wrap and return to freezer for 3 hours or until firm. Turn the sherbert out onto a serving platter and serve immediately.