1. Using a sharp knife, peel the grapefruit and oranges, cutting away all of the white pithy part of the rind. Cut the fruit into segments.
2. Transfer fruit to a medium bowl. Stir in liqueur and sugar; let stand at least 5 minutes before serving to let sugar dissolve. If not serving immediately, cover and refrigerate mixture until needed (up to 1 day).
3. To serve, divide the citrus mixture evenly among 4 small dishes or bowls. Top each portion with a scoop of sorbet.