Crush cranberries in a food processor or blender with 1 Tablespoon granulated sugar. Once crushed, place in a large bowl.
Add marshmallows and sliced grapes to crushed cranberries. Stir to combine.
In a separate large bowl, using an electric hand-mixer, beat whipping cream and remaining granulated sugar until stiff peaks form. Pour whipped cream into the berry mixture, and fold gently until everything is combined.
Cover bowl and refrigerate a few hours or until chilled before serving. Enjoy!