I agree about characters becoming plot devices, but I don't think that's necessarily bad. What you decide to not cover in a story is important. If we are reading a work that focuses on the relation of two characters, going into the backstory to identify why some third character did something is a bit off-point. You can't cover everything.
In this case, what would be gained from knowing definitively 'why' the nuns act as they did? Well, we might learn more about society. We might learn about how Lady Cannibal knows about the fire. We might understand the nuns better. Of these things, I think only the second adds to this particular story(which seems to be about the relation between two characters). In such a case, the knowledge might be kept for later as a reveal.
I think the way that the nuns acted isn't that unusual. Being a nun, catholic, or working at an orphanage are not things that preclude people from treating children unfairly. Rather, I'd imagine each would provide its own reasons for doing so. This complaint just betrays a lack of imagination or the willingness to use it.