Sibling estrangement is not just about not talking to your brother or sister. It has much broader ramifications, as sibling rejection can profoundly shape an individual's personality and their roles in the family.

The estranged may lose the opportunity to be a sibling, in-law, aunt or uncle, and even son or daughter, as estrangement often metastasizes and family members choose sides. These shifting alliances may contribute to greater alienation.

Even worse, sibling estrangement disturbs self-esteem, stamps an individual’s understanding of his or her self, and shapes present-day relationships.

This is not surprising, given that in childhood, brothers and sisters are our first playmates, instilling in one another fundamental social qualities—tolerance, generosity, loyalty—that eventually affect relationships with friends, colleagues, and lovers. Dr. Karen Gail Lewis, author of Sibling Therapy, has identified this concept as “sibling transference”—old childhood feelings toward a sibling resurfacing in adulthood.

“The person may be responding to people in their current life as they did to their siblings way back when,” she writes. “What makes all this so complex is that these behaviors are unconscious; ghosts live within everyone, but they haunt inconsistently.”

A devastating consequence of sibling estrangement can be the cut-off’s disturbance of the ability to trust, damaging other relationships that the estranged holds dear. Someone who has been rejected by a family member struggles with trust in two ways.

First, as documented by social scientist Kylie Agllias, author of Family Estrangement: A Matter of Perspective, the estranged often limit their........

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