Mash jellied cranberry sauce in a 3-quart bowl. Stir gelatin mix thoroughly into cranberry sauce. Pour boiling water into mixture and stir until gelatin has dissolved completely, about 3 minutes. Chill in refrigerator until softly set, 2 to 3 hours. Gently fold sour cream into gelatin mixture, leaving marbled streaks of sour cream and gelatin mix. Return to refrigerator until firmly set, 4 hours to overnight. Store leftovers in refrigerator.