Depends on whether you are going for a particular look, or just good flavor. If just flavor, I would only do the buttercream if it is for a frosting person (I don't usually like fristing, but your sister might love it). If for looks, just the ganache can be pretty, or it can be dressed up with sprinkles, sparkly sugar, or something like that. But if you are going for a certain look, or trying to tie into a theme, you might need the buttercream to accomplish that.