On any given day, you might find 11-year-old Ari’Yanna on the basketball court, while her 9-year-old sister, Ariyah, is at Majorettes practice or sprinting down the track. The girls are social, active, and quick to laugh. In school they seem like any other kids their age…

Which is why people are usually surprised to learn they both have sickle cell disease, a rare inherited blood disorder that can cause episodes of severe pain crises, chronic fatigue, and other serious complications. When friends hear about their diagnosis, or the fact that they receive treatment at St. Jude Children’s Research Hospital, their reaction is usually the same: “You don’t seem sick.” That’s because sickle cell disease is often invisible except to the people living with it. For this reason, the girls keep a close eye on each other throughout the school day, instinctively seeking each other out and somehow knowing when the other is in pain. When one sister senses a pain crisis coming on, she alerts a teacher,........

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