On a morning in September, Jennifer Gates Nassar, 29, arrives at a Manhattan photo studio at 9:30—the first one here, before the photography crew even, for a shoot that will take place later that day with her younger sister, Phoebe, 23, and mother, the philanthropist Melinda French Gates. This is typical, Phoebe tells me later: “She would always arrive early if she could.” When Jennifer was taken to school as a child, her mother adds, she liked to get there just after the teachers.

“I’ve gotten better with time!” says Jenn, now a pediatric resident at Manhattan’s Mount Sinai hospital. Jenn was the kind of oldest child who made PowerPoint presentations to convince her parents that the family deserved a dog. Together with Phoebe and their middle sibling, brother Rory (now an analyst in Wash

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