Add 1-1/2 cups boiling water to 2 pkg. gelatin mix; stir 2 min. until completely dissolved.
Pour into 6-cup mold or 1-1/2-qt. bowl sprayed with cooking spray. Refrigerate 5 min. or until gelatin is thickened. Top with grapes; press gently into gelatin. Refrigerate 20 min. or until set but not firm.
Add remaining boiling water to remaining gelatin mixes; stir 2 min. until completely dissolved.
Pour into blender.
Add condensed milk and sour cream; blend well. Gently pour over gelatin layer in mold.