An award-winning documentarian and host of the popular Relatively podcast delivers the definitive book about siblings—often the longest relationships of our lives, and the least acknowledged.

For many people, relationships with brothers and sisters last a lifetime, spanning decades. Sibling relationships precede friendships or romances and outlast most connections to parents. Eighty percent of us have brothers and sisters who share our DNA and are often the unique keepers of our intimate histories. Our siblings can be our allies and our competition, our tormentors and our protectors, our best friends and our enemies. Research has found that these relationships are just as influential to our development as parenting.

Documentarian and celebrated podcast host Catherine Carr—herself a middle child—became convinced of the profound importance of sibling relationships. In Relatively, she takes us on an unprecedented journey through some of the universal themes of siblinghood: designated roles and labels, friendship and enmity, shared trauma, family language and jokes, and separation and estrangement. Drawing on over fifty conversations she has had with pairs of siblings for her podcast, new research, studies by psychologists, and fascinating depictions in popular culture, she sheds new light on these vastly underappreciated relationships that profoundly affect our lives—relationships that are formative, vital, and full of clues about how to make sense of how we get on with others.

Part journalistic deep dive, part storytelling, part pop cultural critique, this conversational, illuminating, and endlessly absorbing work is a long overdue look at our sibling relationships and how they define us.