Maybe it's something to do with an innocent child asking mercy from the "mother with the empty womb". A child belonging to a woman who is cursed by the witch/mother. The witch, via the curse, is a part of you, and was part of your husband. Therefore, the witch is in essence part of your child, as well. Maybe the witch feels a connection to your little girl. Perhaps, even, a relationship of sorts. Could it be that the connection is possibly thawing your/their frozen heart?

I'm not great at reasoning things out. But I feel like the witch feels some sort of kinship(?) to both you and your daughter. Honestly, I think you should strongly consider at least trying to talk to her. It's simplistic, I guess, but as someone unable to have children, i cant help but wonder if her cold empty mother heart might respond to some love or kindness.

I'm so, so sorry about your parents. One thing for sure, I wouldn't give up that book for anything, now. That thing could have given you the option to give it the book before harming your parents. But no, it just killed them for the heck of it. Screw that monster.