Bring sugar and 2/3 cup water to a boil in a small saucepan over high, whisking occasionally. Reduce heat to low, and cook, whisking constantly, until sugar dissolves, about 2 minutes. Add hibiscus flowers and star anise; stir once to submerge flowers. Simmer, undisturbed, until syrup is light red in color and infused with a floral flavor, about 6 minutes. Pour mixture through a fine wire-mesh strainer into a bowl, pressing solids to extract all liquid; discard solids. Chill syrup, uncovered, until cold, about 2 hours.