Stir together first 2 ingredients in a small bowl; cool completely (about 20 minutes).
Beat whipping cream and gelatin mixture at high speed with an electric mixer until foamy; gradually add sugar, beating until soft peaks form. Stir in sour cream, 1/4 cup at a time, stirring just until blended after each addition.
This is a very springy, light frosting that doesn't leave you in a sugar coma after eating it. My only suggestion is that you make sure you whip the cream to soft peaks, even leaning on the side of hard peaks. I thought I whipped it to soft peaks but then I added the sour cream and it came out very soupy. After chilling it for a few hours I managed to save it.