Mustafa maybe your sister can get some Lumiere Alticolor or Filmcolor, have you tried to contact Frederic Mocellin or Ronan Guinée the later is from the Albert Kahn Collection

Since Autochrome plates are sandwiched between two glass sheet the original plate + a protective plate it's very hard to see wether a plate is varnished or not. Maybe your sister can find a box with undeveloped plates at a marchee au puce, there is a big one specialised in antiquities.

Good Luck