Boil sugar with 125 ml (approximately 4 ounces) of water in a pot, remove from heat then let cool.
Blanch peaches in boiling water in a pot.
Remove peaches from water, drain, peel, halve, pit and cut into small cubes. Puree peaches together with lemon juice and liqueur using an immersion blender and finish in the ice cream machine or freeze in a shallow bowl. If in the freezer, stir every 30 minute with a fork so the sorbet is smooth. Once the sorbet is frozen it can be served. If frozen overnight, remove from the freezer 20 minutes before serving.