There is no way they would have gone to the effort of prepping Diana to have eggs harvested and harvesting them. They barely had time to do so.
At that time, it was cutting-edge medicine, and the prospective Queen of England would not have been offered up as a guinea-pig.
Further, for this woman to be the sibling of Will and Harry, that would mean she preceded the official first baby born from a donated egg in 1983.