So, at this point, I’m sure we can all agree she was going to become the literal sacrifice to transplant all her “dormant” magical powers into another individual, aka the FL of the novel, whom is probably a puppet or very devoted follower of the church. Dad is still an monster for allowing and most likely agreeing to the terms knowing his daughter will die. And maybe they had to keep the powers suppressed not only because they didn’t want anyone to know she’s the reincarnation of her ancestor, but also that the ‘mana’ flowing through her now is utterly useless once it is accustomed to one’s body, they needed a ‘pure’ source that wasn’t tainted by our FL’s mortal body. Then again, still speculation at this point, so no spoiler warning.