Mona Seif said on Tuesday that all contact with her brother, prominent Egyptian activist Alaa Abdel-Fattah, has been cut off completely, as his hunger strike reached day 115.
"Human rights prisoners are being abused everyday in Egyptian prisons, and there is a citizen called Alaa Abdel-Fattah, an Egyptian-Briton…all means of communication with him have been lost and we don’t know anything about him," Seif said furiously in a video posted on her Twitter account.
She lashed out at the British government for not doing more to help with her brother’s case.
Alaa was sentenced in December to five years in prison after he was convicted along with two others of "broadcasting false news".
Abdel-Fattah gained UK citizenship in April from inside prison, through his British-born mother.
"I’m not going to even talk about the British Foreign Minister Liz Truss, who should be intervening to help protect Alaa at the moment, but she’s too busy at the moment in the race to become England’s next prime minister," Seif said.
Truss said in June that Britain was "working very hard to secure his release".
"There’s a mother right now called Leila Soueif sat at the door of the…prison, waiting for a handwritten letter from her son to tell her that he’s alive and what’s going on."
Mona Seif told Egyptian authorities that for years her and her family have been harassed, and that they were willing to leave Egypt straight away if they handed over Alaa, but warned that if he died in prison they would not drop the case.
"What do you want from us, to glorify the president?! That will not happen! We will not glorify a person who is responsible for violating human rights on a daily basis!" Seif added.
She demanded that her and her mother be given reassurances on Alaa's safety.
Egypt's interior ministry said last month that it had footage that "disproves" reports of his hunger strike.