Whatever happened to the nuns that were assisting Mildred in the original Arcana Heart? After Mildred is defeated, she gets sucked into the Arcana world, and her floating castle vanishes. Either the nuns got sucked in along with her, or it's possible they all died.
Pistrix's official designation is 'Series Pistrix No.29'. Which means there were at least 28 previous attempts at creating a homunculus by Parace L'Sia, and it's likely they were all designated failures and disposed of.