Refrigerate a medium-size bowl and beaters until very cold. Combine cream vanilla, brandy and Grand Marnier in the bowl and beat with electric mixer on medium speed 1 minute. Add the sugar and sour cream and beat on medium just until soft peaks form, about 3 minutes. Do not overbeat. (Overbeating will make the cream grainy, which is the first step leading to butter. Once grainy you can't return it to its former consistency, but if this ever happens, enjoy it on toast!)