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A family of five silhouetted in front of a sunset.

The first Facebook message arrived when Heather Woock was packing for vacation, in August 2017. It was from a stranger claiming to be her half sibling. She assumed the message was some kind of scam; her parents had never told her she might have siblings. But the message contained one detail that spooked her. The sender mentioned a doctor, Donald Cline. Woock knew that name; her mother had gone to Cline for fertility treatments before she was born. Had this person somehow gotten her mother’s medical history?

Her mom said not to worry. So Woock, who is 33 and lives just outside Indianapolis, flew to the West Coast for her vacation. She got a couple more messages from other supposed half siblings while she was away. Their persistence was strange. But then her phone broke, and she spent the next week and a half outdoors in Seattle and Vancouver, blissfully disconnected.

It was only when she got home and replaced her phone that she saw the barrage of messages from even more half siblings. They had found her on Facebook, she realized,...