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.