When Your Ancestry Test Entangles Others

Stephen Wald took a home DNA test in 2018, hoping to explore his family ancestry with his two young children. Instead, he learned a family secret: The man who raised him was not his biological father. Several days of intense online sleuthing for more details yielded another surprise, the identity and phone number of an apparent half-sister, Julie Gale.

Mr. Wald, then 60 years old and a New York City-based real-estate broker, was well aware of how it felt to be unmoored by DNA test results. “I knew I was delivering big news,” he says. But he didn’t hesitate to make the call. “There was no ‘Should I not do this?’’’ he says. “I was on a mission. That was it.”

Only later did it fully hit Mr. Wald that, in his quest to discover his own origin story, he might disrupt the narrative of someone else’s family—or disturb other members of his own.

Consumer DNA testing has taken off in recent years. Millions of people have spit in a tube or swabbed their cheeks, enticed by prices under $100...