Muharram Tojiboeva told RFE/RL's Uzbek Service she tried to visit her sister today at the Tashkent women's prison colony where she is serving an eight-year jail term.
She said prison officials told her that her 44-year-old sister and one of her codetainees were being kept in a disciplinary cell.
She said her sister is accused of undermining Uzbekistan's constitutional order.
An independent Uzbek website, muslimuzbekistan.net, on October 16 reported that Mutabar Tojiboeva had managed to pass a note to relatives in which she said her health was deteriorating and that she was suffering from hunger.
A rights activist from the eastern Ferghana Region, Mutabar Tojiboeva was arrested in October 2005 as she was preparing to attend an international conference in Dublin, Ireland.
In March, a court sentenced her on charges of blackmail, charges her relatives say were fabricated.