1 (.25 ounce) envelope unflavored gelatin
3 cups water
4 cups cranberries
2 cups white sugar
1/2 cup lemon juice
6 fluid ounces vodka
3 fluid ounces triple sec
1 ounce lime juice
8 ice cubes
Stir gelatin into 3 cups water in a bowl until dissolved.
Place cranberries into a large pot and cover with water; bring to a boil. Reduce heat to medium-low and simmer until skins pop open, 10 to 15 minutes. Drain cranberries through a sieve.
Combine cranberries, gelatin-water mixture, and sugar in a pot; bring to a simmer. Cook and stir cranberry mixture until sugar is dissolved, 5 to 10 minutes. Set aside to cool; stir in lemon juice.
Pour cranberry mixture into a container and cover; freeze 8 hours or overnight.
Blend 2 cups cranberry sorbet, vodka, triple sec, lime juice, and ice cubes in a blender until smooth. Pour cosmopolitan mixture into 4 martini glasses.