Okay kids, get aboard the time machine:
In chapter 1 Charlotte was already coughing blood. Talk about one of the reasons she decided to finally rest in peace
In chapter 3 Jeffrey mentions the curse, and that he should have stopped her that day.
In chapter 8 he talks about curse again. That it's still in place.
My best guess is it's the curse they both got that day at the very beginning of the story. It applies to them both at the same time, and it's reactivated because that horned guy has returned.