Diana's gown was based on the ones worn by her mid 18th century Spencer-Churchill ancestors in the Gainsborough portraits at Althorp House.
Those were the dresses worn during the period of Charles II -Queen Anne.
Therefore it was way before the Victorians and looked nothing like clothing from that era.
Sarah's was Victorian.. Diana's was a bit of a mix of "fairy tale princess, and earlier and Victorian. her little bridesmaids were semi Victorain...