Segment grapefruit and oranges over a bowl to catch juices. Set segments aside. In a medium saucepan, stir together the sugar and gelatin. Measure citrus juice, then add enough orange juice to equal 1-3/4 cups. Add juice to saucepan. Cook and stir over medium-low heat until gelatin is dissolved. Remove from heat. Gently stir in citrus segments. Immediately pour into trifle dish as directed in Layer 1.
LAYER 1 Pour citrus gelatin into a shallow 9- to 10-inch diameter, straight-sided clear glass bowl or trifle dish. Refrigerate least 3 hours before adding remaining layers.
LAYER 2 Stir together sour cream and the milk. Spread on gelatin layer.
LAYER 3 Spread cherries on sour cream layer.
LAYER 4 Top cherry layer with marshmallows.
LAYER 5 Sprinkle with coconut, then top with stemmed maraschino cherries, if desired. (If assembling well ahead of serving, wait to add final layer until time to serve.)