So clearly the pills are doing something odd to her, and here's my theory. That saintess woman's powers and/or spirit likely reside in Aisha due to her bloodline. However, these can only be unleashed upon Aisha's death, and once that happens (maybe after a time), she resurrects not only as the saintess but also as Aisha, merging their bodies/counsciousness to become Deborah from the story. I think the pills are some kind of magic blocker, preventing the immense amount and power of her own mana from consuming her life to allow for the transformation. So the dad is trying to keep her alive, the church is probably corrupt and trying to keep the saintess from coming back, and if Aisha stops taking the pills then she'll either die or be able to use magic on the level of Deborah/saintess.