Late in his life, George H.W. Bush pondered just who he would want to see first in heaven.

The nation's 41st president took the question posed by granddaughter Jenna Bush Hager in an interview for NBC’s "Today" seriously, turning it over in his mind. If Barbara Bush, his wife of seven decades, died before he did, he would want to start with her, he decided. And the mother he adored, Dorothy Walker Bush.

And Robin.

He had a picture in his mind’s eye of how he hoped his daughter would appear to him – as the chubby, vivacious 3-year-old she had been before she was diagnosed with leukemia in 1953. Until that day, he had never heard of the disease that would kill her six months later.

The illness and death of Pauline Robinson Bush would be threaded through the life of her father and her mother. The experience taught them a terrible lesson about the ways the innocent can be caught and crushed by life's unfairness. It left a stamp about what matters, and what doesn’t. It fueled George Bush's determination to do something big in life, beyond the oil business he was building in Texas. After he became president decades later, it helped shape his policies toward the epidemic of HIV/AIDS.

Robin’s illness tested her parents’ marriage, then strengthened it.

The young couple’s response to that crisis forged a template they followed through the ebbs and flows of their long union, at 73 years the longest of any president and first lady in U.S. history.

One rule: No crying

When Robin was being treated at New York's Sloan Kettering Memorial Hospital, Barbara Bush set one rule: No crying in her room. Her mother didn’t want the little girl unsettled by seeing the adults in her life in tears.

George Bush, a man of open emotion, found it almost impossible to comply. Again and again, he told Robin he had to go to the bathroom, then stepped into the hallway to regain his composure.

“We used to laugh and wonder if Robin thought he had the weakest bladder in the world,” Barbara Bush said later. “Not true. He just had the most tender heart.”

George Bush knew how hard it was on Barbara to be the stoic, to be the one in control. Years later, he wrote a revealing aside in a letter to a constituent in his congressional district who had been diagnosed with cancer. “Someone had to look into Robin’s eyes and give her comfort and love,” he said, “and somehow, Paul, I didn’t have the guts.”

When Robin died, Barbara was the one who collapsed into sorrow, and George Bush was the one who took charge. “Time after time during the next six months,” she said, “George would put me together again.”

The pattern of one stepping up when the other was struggling – and of being able to switch those roles between them – sustained the couple during times of political defeat and personal pain from then on.

That was the defining experience Barbara Bush chose to talk about at the Republican National Convention in 1988, a moment of political triumph as her husband was nominated for president. She noted in passing her husband’s impressive résumé, the stints in Congress and at the United Nations and the CIA and the vice presidency. Then she turned to their personal tragedy.

“The hardest thing we ever faced together was the loss of a child,” she said, her voice thickening as she worked to keep her emotions in check. “I was very strong over the months we were trying to save her – at least, I thought I was. Maybe I was just pretending. But when she was gone, I fell apart. But George wouldn’t let me retreat into my grief. He held me in his arms, and he made me share it and accept that his sorrow was as great as my own."

The stigma of disease

During her treatment in New York, Robin was allowed to go home to Midland, Texas, for one last visit. Even some family friends were afraid to visit, worried that the then-mysterious disease might be catching.

“When Robin got sick, people avoided us like the plague,” Barbara Bush said in an interview last year with this writer for a biography. Her voice was marked more by sorrow than anger, all these years later. “Even my close friends.”

When Bush became president, HIV/AIDS faced a stigma of its own, one laced with discriminatory attitudes toward homosexuality. The Reagan White House had done little to address the emerging crisis.

Bush delivered a speech on AIDS in March 1990, his first major attempt to address the subject. He called for the National Institutes of Health and the Centers for Disease Control to go “on a wartime footing” to combat the disease.

“When our own daughter was dying of leukemia, we asked the doctor the same question every HIV family must ask – why, why this was happening to our beautiful little girl?” Bush said in the speech. He had seen friends die of AIDS, he went on. “There is only one way to deal with an individual who is sick: with dignity, compassion, care, confidentiality, and without discrimination.”

It was the only time during his presidency that Bush mentioned Robin by name in his public remarks. In the privacy of his diaries, he sometimes noted her birthdays. In affectionate letters to his wife, he repeated a phrase that Robin coined: “I love you more than tongue can tell.”

On the day in October 1987 that he announced he would run for president, the election that would cap his career, Bush was thinking about the daughter he had lost. “Bar looks beautiful,” he wrote in his diary, then added, “Thirty-four years ago today, Robin died.”

After a memorial service at the National Cathedral in Washington and a funeral at St. Martin’s Episcopal Church in Houston, George Bush will be laid to rest in a wooded grove near his presidential library in College Station, Texas. He will be buried Thursday beside the grave of Barbara Bush.

And next to Robin.

Susan Page, USA TODAY’s Washington Bureau chief, is the author of "The Matriarch: Barbara Bush and the Making of an American Dynasty," which will be published in April 2019.