Combine 1/2 cup sugar and 1/2 cup water in a small pot and bring to a boil over medium heat. Boil 1 minute, swirling the pot occasionally, to make sure the sugar is dissolved, then transfer the syrup to a heat-proof container. Cool. The simple syrup will keep for a few weeks sealed in the refrigerator.
Put the grapes and 2 tablespoons sugar in the bottom of a large serving pitcher and muddle to crush them. Stir in the grape juice and brandy, and then the pinot grigio. Taste, and add the simple syrup to sweeten, as desired.
Stir in the mint and lemon, lime and orange slices. Chill for at least 4 hours. Serve over ice in large wine glasses.