Dissolve both gelatins in boiling water. Beat in sour cream with a wire whisk or rotary beater until smooth. Stir in sherbet until smooth. If necessary, chill until partially set. Add cranberry relish. Turn into a 5-cup mold.
Cover and chill 4 to 6 hours or until firm. Unmold onto lettuce-lined platter. Garnish with sugared cranberries, if desired. Makes 8 to 10 servings.