I give the book a high rating because the story is told so well, backed up by solid narration - I felt as though I were (vicariously) living the events as they unfolded. At the risk of re-hashing the plot (something I frown upon generally), Federico's mother and step-father have managed to "fake" their way along as their mental health deteriorated; her mother was losing her sight as well. On vacation in Florida, however, Addie suffers a traumatic episode, triggering a "crisis" situation, until her death (it wasn't clear to me how much time elapsed, a couple of years perhaps). Addie and Walter had been married and widowed, bringing children to the union, but The Brady Bunch this is not - toleration, not cooperation, best describes the kids' relationship; he had sold his house and moved into hers after the wedding.
Getting Addie home to New Jersey was a story in itself (Walter's daughter had taken him back alone shortly after the incident). She's placed in a facility, which ... doesn't work out. Various health aides are hired in shifts to look after the couple at home from then on, with the author and her siblings doing their best to "supervise" things long-distance. The step-sister, who lives nearby it seems, is implied to do as little actual work as possible. She's mentioned only a couple of times, once as she had "taken her father to New York for the day in a limo." The implication being that she's lazy and self-centered.
Meanwhile, he becomes more verbally and physically abusive, to everyone in the house, including Addie. The author mentions in a short aside that she was advised having (them) declared incompetent would be too difficult to pursue. Having some experience with such things in New Jersey, she should've moved her mother into a facility, citing the documented problems. Later on, there's a memoir-ish section on Addie and Meg's backgrounds, which was useful in putting the story in perspective.
Final thoughts ... Meg was either as naive (I hesitate to use the term "clueless") as she maintains, or a bit of a martyr. That may sound harsh, but if I had a physically abusive stepfather like that, I'd have gotten my mother out of there - or at least tried, in spite of the pessimism of those she says consulted. That having been said, I recommend the book for the writing, and narration - one of those cases where I'd say the audio seems preferable to reading the printed version.