I actually rather liked the original theme of the mother having been dead for some years, barely remembered by the kids and only related through their father's memories.

Maybe, in this version, so that Holly would think that she was seeing her mother, have Rick explain that their mother vanished for a few days, mysteriously, before being found by the police, in the woods several miles away from home. Dying a few days later, she was never able to relate what really happened to her.

A thought1