Yes the RF could have taken her parents in hand, taught them etiquette, kitted them out in suitable clothes then we could all be here saying How patronising!
His older children bear a lot of responsibility for this fiasco, since it became known that they were an item the half sister in particular has been money-making mad, never off the TV or out of the sleazy magazines. Even her father is discussing his plans with a celebrity trash-site, how does anyone know why she hasn't seen him? Maybe he didn't want to be part of the media circus surrounding the wedding although he seems to have embraced them later.
I feel sorry for Harry, I would bet that he would pull out of this if he could.