That witch's identity is extremely obvious and it's not the demon girl. The demon girl would have no reason to want to punish Yvonne for making Elsa wait during the festival, for example.
Interesting. If i remember correctly the nun was a demon as well, or do i remember wrong? Anyways, from what I took from this chapter is that only the sage knew purification magic. So is the nun olivia? Considering that she was raised as an orphan by a saint and is no walking in her mothers footsteps, kind of. Maybe she felt guilty as she probably caused lenas death.
Now I'm gonna ask the real question. How is Yvonne part Demon then? In the cloister/orphanage arc we got to see that Yvonne is really reall susceptible to it, so we can infer she's part demon, where did that come from? @.@