Poach the rhubarb in the water with the caster sugar until soft. Strain the liquid off into a jug. Keep some of the rhubarb for serving.
Warm the strained rhubarb liquid and stir in the softened gelatine sheets or powdered gelatine until dissolved. Stir in the vodka and Cointreau, then strain into 4 glasses. Chill until set. Decorate with some of the poached rhubarb.