Boil sugar with water to syrup and let cool. Squeeze grapefruit and lemon (approx. 200 ml) and pour through a sieve. Pour in the white wine and Campari. Beat the egg whites until stiff and fold into the juice. Put in a metal vessel and let freeze. Stir every 30 minutes until the liquid is frozen.
Spread over sorbet glasses and serve poured over with sparkling wine.