Spray a 6-cup gelatin mold or bundt pan with nonstick cooking spray. Place the gelatin in a medium bowl.
Strain the pineapple and mandarin oranges, reserving the liquid in a large measuring cup. Set the fruit aside. Add enough water to the fruit juices to equal 2 cups. Transfer the juice mixture to a small saucepan and bring to a simmer over low heat. Pour the hot juice mixture over the gelatin and whisk to dissolve. Add the orange sherbet and stir until fully melted and combined.
Add the sour cream and beat with an electric mixture until smooth, 1 to 2 minutes. Pour the mixture into the mold and add the reserved mandarins and pineapples, making sure to distribute the fruit evenly throughout. Refrigerate until fully set, at least 3 hours up to overnight.
Carefully dip the mold in hot water for 15 to 30 seconds. Remove and place a platter over the top of the mold. Invert onto the platter and serve.
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