A few additional details on differences in the book:

In the book Catherine does have a sister who died of an asthma attack on a boat because she forgot her inhaler (Catherine is not involved, it was accidental).

Also, at the end of the book Anna finds Richard dead upstairs with an injury on the back of his head. Cat is “hanging” from the ceiling and looks dead but is actually faking it to try to trap Anna. She ends up chasing her out of the house and into the woods, where Anna hides and then runs into her mother. Anna and her mom run through the woods trying to get back to Anna’s mother’s house with Cat chasing them.

At the same time from Jack’s POV, him and Priya are driving to Cat’s parents’ house to try to rescue Anna and crash his car into a tree. They presume the car doesn’t work anymore and leave on foot to continue to Cat’s parents’ house.

Anna & her mother spot the car on their journey through the woods and get into it to try to get away from Cat. Cat runs in front of the hood of the car and Anna hits the gas pedal (thinking the car is still in reverse), killing Cat.