Using a sharp paring knife, cut the peel and all white pith away from the outside of the grapefruit and oranges. Working carefully over a serving bowl, slip the knife along the membranes on both sides of each segment to release the segments from the membranes into the bowl. Repeat until you have removed all segments from the citrus fruit. Squeeze the membranes to release any juices into the bowl. Remove the pits from the nectarines, then cut each half into 8 thin slices. Peel the kiwi and cut each into 8 to 10 thin lengthwise slices. Add the mint and sugar and toss gently to combine. Refrigerate for 15 minutes before serving. Toss gently and serve in small chilled bowls.