The latest move coincides with the high-profile release of Chinese tech executive Meng Wanzhou and two Canadians over the weekend. Critics have in the past accused China of using ordinary citizens as political bargaining chips, in what is known as "hostage diplomacy". China has always denied this. On Monday, US senators Edward Markey and Elizabeth Warren said in a statement that Cynthia and Victor Liu were back home "after three difficult years being held in China as pawns for the Chinese government".
They added that the siblings' mother, Sandra Han, was also a US citizen and was "detained in China". The senators said they were working with the US government to secure her release.