You know, I just thought of something.
Though it was the Crazy girl who was after the Main girl. But the one behind the Crazy girl was "that Being" which gave her directions on what to do to achieve her desires.
What if "that Being's" goal was to stop Main girl from marrying and having descendants in the first place?
And now that Main girl had become a Nun/Sister, she is practically following the path of celibacy, so it has achieved its goal and now going to target someone else (paving way to MC Dragon's next target to protect)
And this wouldn't stop unless this world has the system of hybridization/cross-breeding or MC Dragon can take human form.