From what I gathered it was because the duke only married her because Bunnie was in danger. She convinced the duke she needed to be with them to keep Bunnie "the symbol of peace" safe. I do believe she is the one who either planted a fake or just convinced them there was another Bunnie they are "protecting"
Wait... So Bunnie is the duke's daughter?
No that's not it.
I de believe I read somewhere that she is indeed the real one. Spoilers I guess
I don't understand what the saintess's problem is. Like what do you want? Yeah, I get you want to kill Damien because you are the empress's lackey or something but why Bunnie? I don't understand her actions at all. She does stuff for no reason. She has no threats in the Dutchy, nor does it seem like she wants power from the Dutchy(cause if you want power you wouldn't mistreat Bunnie. You would try to attack the boys or kill Bunnie to bring out Ophelia) Her character is really confusing.