1. Slice nectarines into a bowl. Add lemon juice and mix with fruit to keep slices from darkening.
2. Pour fruit and juice into a blender or food processor. Add 2 tablespoons sugar, liqueur, vanilla, and sour cream. Whirl until peel is finely ground (it turns the purée pink). Taste and add more sugar if desired.
3. Pour nectarine mixture into a 1-quart or larger ice cream maker (refrigerated or frozen-cylinder kind, or one that uses salt and ice) and follow manufacturer's directions. Churn until dasher is very hard to turn.
4. Scoop nectarine sorbet into bowls and serve.