Strain the mixture through a metal sieve and while the syrup is still hot, dissolve the gelatin (swollen beforehand) in it. Stir until dissolved and put it in a bowl of icy water to cool, while you stir once again.
To the cooled raspberry syrup, stir in the sour cream until the cream homogenizes. Finally, add the whipped liquid cream.
Distribute thus prepared raspberry mousse in dessert cups or bowls and put them in the fridge for a few hours to harden.