As a clinical psychologist, Mary E. Plouffe has counseled many people who deal with the grief caused by the death of a loved one. Her memoir, “I Know It in My Heart,” tells of a time when death and grief settled in the midst of her own family. The lost loved one is her younger sister, Martha, who is diagnosed with an aggressive cancer and embraces an experimental therapy that might give her the chance to see her 3-year-old daughter, Liamarie, grow up.

Plouffe’s memoir is poignant and heartbreaking. It opens the door to what is fundamentally a most private journey into and through trauma. Plouffe, painstakingly and with keen insight and heart, invites the reader on that journey. It is a story with many parts. These include her sister’s journey, Plouffe’s own story, and centrally, the journey of Liamarie.

Plouffe and her sister Martha couldn’t have been more different. “There were three and half years between us,” she writes, “but personality separated us more than age.” The author grew up as an introvert, a “watcher of all the Irish subtext” in her father’s extended family. She was self-conscious and moody. Martha “had her own agenda, and couldn’t care less what adults were doing unless it interfered with her plans.” She was a natural comic and happy to take center stage.

Martha was also a radical activist and often scolded Plouffe for not taking politics more seriously. Martha and her husband, Herb, were so politically involved that it surprised the family when she became pregnant with Liamarie, who was born of delicate build. “But she was whip fast and quick with words, startling those who heard her,” Plouffe writes. “She spoke full sentences at two, with extra words thrown in whether she was sure of them or not.”

The heart of the story begins with a phone call from Martha in January 1997, telling her sister she has a tumor. The two women, who had never been especially close, are drawn tightly to one another, with Plouffe becoming her sister’s confidante and sounding board, and Martha becoming a most precious friend. Martha ends up opting to pursue a promising treatment involving bone marrow transplants at Johns Hopkins Hospital. Plouffe and her husband, Bill, agree to take Liamarie into their home whenever necessary to accommodate the treatment, as Martha’s husband has to travel for business. The Plouffe immediate family circle includes Plouffe – who Liamarie calls “Mary Beth” – Bill and their three children, fifth-grader Margaret, 15-year-old Matt and 20-year-old Justin. On the periphery are Bill’s mother, and Plouffe and Martha’s mother, both elderly and in deteriorating health.

Plouffe is masterful at storytelling and brings Liamarie alive on the page. Early on, Martha explains to Liamarie, “‘Papa has to work in Las Vegas, and I need to go to the hospital for a few weeks. So you will stay with Mary Beth and Bill in Maine.’ At first Liamarie resisted in her usual self-assured way. ‘No Mama. I don’t think I will do that. I will stay with you.'”

Plouffe is also generous in giving readers a basic, yet rich understanding of how 3-year-olds view and respond to the world. “Three-year-olds also have little internalized visual or verbal memory,” she writes. “So, it is not the thought or picture of Mom that brings feelings of separation and sadness. Rather, sensory memory rules, and sensation triggers loss. A hurt, a fall or the feeling of crawling into bed, associated with a mother’s warm touch – all these bring out the craving for Mom.”

The story tells of momentary delights of having a precious 3-year-old join a family. Liamarie easily bonds with Plouffe’s children, but especially with Margaret, with whom she shares a room. But she struggles to fathom her two older cousins. Matt, who is in high school, is always coming home and then leaving again to go to sports practice, to be with his friends or to play music. Liamarie wants to know why. Plouffe attempts to explain, but it still makes no sense to Liamarie. Plouffe tells her, “‘He’s a teenager, Liamarie,’ an explanation I offered with a shoulder shrug that implied this was a mysterious state that could not be understood. She adopted that gesture and phrase with a mix of awe and annoyance… ‘He’s a teenager. There he goes again.'”

The memoir deals with the trauma of Martha’s illness in detail. It is stressful and dispiriting for everyone. When it becomes clear that the experimental treatment is failing and Martha lapses finally into a coma, Plouffe, her husband, and Liamarie’s father debate whether Liamarie should be given the opportunity to see her mother in the hospital for what likely would be the last time. “The visual impact in Martha’s room was overwhelming, even to an adult. If we brought her in, she would see her mother bald, eyes half open but unseeing, arms struggling against restraints, legs pushing and pulling as if she were trying to walk away in place. Tubes and IV lines emerged from every corner of the covers. The bright blue vent tube protruded from her mouth…”

They finally decide to ask Liamarie what she wants. The scene was explained to her. Liamarie considers it, “then said softly, ‘No, it would be too scary for me.'”

The entire book is great testament to the suffering and resilience that comes after the death of a loved one. But the section on grief is profoundly provocative. In a chapter called “Condolences,” Plouffe deals with the universal awkwardness that people in this culture have around acknowledging death. The worst of all strategies, Plouffe writes, is avoidance. “When you ignore me, I become invisible altogether… No, see my shadow, name my pain, even if I cry, or bring you sadness. It makes me feel I still exist.”

The last section, “Growing,” marks the passage of Liamarie into adolescence and the continuation of life in the extended family, chronicling the ordinary and extraordinary, including graduations and marriages.

The trajectory of Plouffe’s story falters for having what are essentially two endings. One is for the story that is told, the other for one that isn’t. The last formal chapter of the book tells of Liamarie’s quinceañera, an important, joyous celebration with roots in Latin America of a girl’s 15th birthday.

Then, the epilogue, focused on her sister’s course of treatment, begins: “There is part of our story I have not told. I cannot weave it into the narrative because I am not sure if it is true. That’s not true. I know that the pieces are true.”

The second ending causes the story to falter because Plouffe has not adequately prepared for including it, and it dramatically disrupts the arc of the story that she – and the reader – have already invested in. Early in the book she writes, “Traumatic memories break all the rules. They do not sit logically sequenced in a familiar pattern of beginning to middle to end.” But that theme is not threaded through the story, so that when the reader reaches the epilogue, it is a jolt. As a result, it mutes what is otherwise a wise and moving story.

Correction: This review was updated at 2:55 p.m. on May 21 to correct the book’s title in the headline.

Frank O Smith is a Maine writer whose novel, “Dream Singer,” was named a Notable Book of the Year in Literary Fiction in 2014 by “Shelf Unbound,” an international review magazine. His novel was also a finalist for the Bellwether Prize, created by best-selling novelist Barbara Kingsolver “in support of a literature of social change.” Smith can be reached via his website