Mist a 10-inch decorative mold or Bundt pan with nonstick cooking spray.
In a large bowl, mix strawberry gelatin with 2 cups boiling water; stir to dissolve. Add pineapple with its juices, bananas and strawberries. In a small bowl, mix 2 Tbsp. boiling water with plain gelatin; stir to dissolve. Stir into strawberry mixture. Pour half of mixture into mold; chill until firm, about 2 hours. Leave other half of mixture in bowl at room temperature.
When refrigerated mixture has hardened, fold sour cream into remaining mixture and pour carefully on refrigerated mixture. Refrigerate until firm, at least 3 hours. When ready to serve, place a plate over mold, invert and tap pan gently until gelatin drops out. Serve immediately.