Why, if she's a ghost, didn't Helen look through the house to try and find her parents' bodies and reunite with them over the hundred years (hundred-plus in the graphic novel)? Because she's perpetually trapped in the mindset of being seven years old. She had just fought with her stepfather and caught the curtains on fire before running out and drowning in the pond. Even as a hundred year old ghost, she was still a child when she died and was locked into several child mentalities, including continuously blaming herself for starting the fire and believing they were mad at her for it. She likely had no idea where exactly in the house her parents died. She may have even thought they had been found in the wreckage and were buried elsewhere while she was found in the pond later, hence her thinking she'd been abandoned.
If Molly hadn't saved Heather from drowning in Harper Pond, the entire family's relationships with each other would be completely destroyed.
Dave would almost certainly divorce Jean and blame her, Michael, and/or Molly (or maybe even all of them) for what happened to his daughter. There are two possibilities—the first one is that he thinks one of them was supposed to be watching her and wasn't, and she ran off and drowned in the pond. The second one is that, given that he thinks Michael and Molly are horrible bullies to his daughter and love tormenting her, he might come to the conclusion that they murdered her. (Even if it's not a rational thing to accuse someone of, grief can do crazy things to people—especially a man who has lost his first wife and then his daughter.)
While Jean would probably not believe that Michael and Molly murdered Heather, there's a possibility she would believe that negligence on their part led to Heather drowning.
Even in the unlikely event that Dave didn't blame her for Heather dying, Molly would be forever traumatized knowing that a ghost led her little stepsister to her watery grave, and any attempts to tell anyone the truth would make people think she was either lying or insane, maybe even that she made it up as a way of coming to terms with Heather's death.
Multiple children have drowned in Harper Pond over the past hundred years, almost always young, lonely little girls—with the parents having no idea they were lured by a ghost. Mr. Simmons describes his cousin Rose as isolated and friendless before she died. Imagine your lonely, quiet daughter goes missing and is found later dead, drowned in the nearby pond. Parents could easily think their child had been Driven to Suicide from loneliness—or worse, lured off and murdered by an adult.
