ISTANBUL (AP) — The two women in the photograph were smiling, but Halmurat Idris knew something was terribly wrong.

One was his 39-year-old sister; standing at her side was an elderly woman Idris did not know. Their grins were tight-lipped, mirthless. Her sister had posted the picture on a social media account along with a caption punctuated by a smiley-face.

"Look, I have a Han Chinese mother now!" his sister wrote.

Idris knew instantly: The old woman was a spy, sent by the Chinese government to infiltrate his family.

There are many like her. According to the ruling Communist Party's official...