Mix raspberries with sugar and lemon juice, heat until the berries are soft and strain mixture through a sieve, so that all the seeds are removed.
Mix raspberries with the raspberry liqueur and vanilla sugar. Beat the egg whites until foamy with a mixer and add the raspberries. Freeze the mixture in an ice cream maker while stirring.
Alternatively, place the mixture in a shallow metal bowl and freeze until firm, 3-4 hours, stirring every 20-30 minutes with a fork to prevent the formation of large crystals. Scoop sorbet and serve in dessert bowls.