Catherine, the Princess of Wales’ sister Pippa Middleton’s backless silk gown worn to the 2008 End of Summer Ball at Berkeley Square was her slinkiest red carpet look ever- see unearthed photos Catherine, the Princess of Wales’ sister Pippa Middleton’s backless silk gown worn to the 2008 End of Summer Ball at Berkeley Square was her slinkiest red carpet look ever- see unearthed photos