ROSSLYN, Va. — Speaking last month at a Washington think tank, Rushan Abbas relayed tales of suffering she had heard about China’s repression of ethnic Uighur Muslims — including the detention of members of her husband’s family in a widespread system of mass internment camps.

Within six days, Ms. Abbas’s ailing sister and 64-year-old aunt disappeared from their homes in northwest China. No family members or neighbors have heard from them in more than a month.

Ms. Abbas is an American citizen and Virginia resident; her sister has two daughters, and both live in the United States. They all assume the women are being detained in the camps, which Western analysts estimate hold up to one million people.

Ms. Abbas said they had fallen victim to the persecution against which she had been campaigning — and because of her.