Combine first 4 ingredients in a large bowl. Cover and chill 8 hours. Remove orange; discard. Press berry mixture through a sieve over a bowl; discard solids. Combine berry mixture, juice, and sugar in an 8-inch dish, stirring until sugar dissolves. Cover and freeze 45 minutes. Stir berry mixture with a fork every 45 minutes until completely frozen and slushy (about 4 hours).
Remove berry mixture from freezer; scrape mixture with a fork until fluffy. Garnish with orange slices.