We can come up with a word or two for Jennet Humfrye, but "sane" isn't one of them. In The Woman In Black, her anger and despair actually make her go crazy—so crazy that she wants to destroy everyone. And confronting the reality of her ghost makes Arthur go a little crazy, too, or at least feel like he's going crazy. Wandering around a big haunted house will do that to a guy.
As Arthur stays at Eel Marsh House, he starts to lose his grip on reality, mirroring Jennet Humfrye's deterioration.
Jennet Humfrye's madness keeps her trapped at Eel Marsh House. She went so crazy following Nathaniel's death that she could not rest even in death.