Wasn't the whole moral dilemma from the Nun's point of view about making peace with the demons lay entire around the fact that human children would essentially have to be sacrificed to allow the demons to be able to remain civilized and not just monsters?.
I guess the other Idaten might be cool...
I don't see why they need to go through the sister to do that. They could always get the daughter pregnant, the mother gets some of those drugs that make someone look a bit pregnant, then do a whole switcharoo the same way.
"Something you only show to someone special to you" "He was my first"
So they were basically doing Alien Sex from the start, meaning Mana got NTRed from since the first chapter.