Blue Sisters rocked the New York Times hardcover bestseller list as soon as it was released last month. Nobody Wants This renewed for a second season barely two weeks after launching. Both phenomena feature sisters—and the relationships between them—as main plotlines.

Most relationship types have been well-studied in psychology, perhaps ad infinitum. Parent-child bonds (Did I hear, “Oedipus,” or “Electra”?), romantic partners, and even “alloparents” have gained ground in social science research. That’s all good but, strangely, siblings have not been a major research focus. And many existing sibling studies don’t even differentiate sibling dyads by gender.

Recent research begins to address this, and with good reason. Siblings can be critically important throughout life. Ask anyone who has one. For me, growing up with three older brothers sometimes gave meaning to Nietzsche’s famous, “What doesn’t kill me makes me stronger.” I didn’t have sisters, though, and neither did either of my parents.

My maternal grandmother and her sister, Sadie, provided my model of sisterhood. It was........

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