The London-based Islamic Human Rights Commission said Nurilhak and Nurcihan Saatcioglu have been arrested several times in the past four years after they attended an anti-hijab ban rally in 1999.
The pair were taken back into custody on Saturday after the Turkish appeal court reimposed an extended sentence for their participation in the rally in Malatya.
They are currently being held indefinitely in the Bakirkoy Woman and Child Prison in Istanbul.
The sisters’ mother, Huda Kaya, has served three years in prison for the same offence as her daughters. A third sister, Intisar, is also due to be arrested for the same charges.
The four women were originally charged with attempting to overthrow the Turkish government, but the charges were changed to “violating the gathering and demonstration act”.
Turkish law upholds a ban on the hijab in universities, higher educational establishments and Islamic colleges.
Harassment of women workers who wear the headscarf is also common in public offices, hospitals and government buildings.
“This supposedly Islamist government has made it clear that reversing the hijab ban is not a priority… I think this is somewhat surprising coming from a prime minister who was himself the victim of religious discrimination”
The nation’s secular custodians say adherence to the headscarf undermines the non-religious nature of the country, and incites religious differences and ethnic tensions.
However, Arzu Merali, director of research at the IHRC, called for the hijab ban to be lifted.
She said: “These women are being harassed for religious reasons. Since the 1997 Turkish army coup Muslims and Islamism have become public enemy number one.
“Forty thousand women have been excluded from university in Turkey because of the hijab ban and I think it is just really stupid.”
She added: “The Turkish people have demonstrated what they think about this issue – in 1998 and 1999 a million people demonstrated against the ban in the streets. But I think they may feel intimidated about doing that now.”
And Ms Merali slammed Recep Tayyip Erdogan’s government for doing nothing to improve the plight of Turkish Muslim women.
“This supposedly Islamist government has made it clear that reversing the hijab ban is not a priority,” she said.
“I think this is somewhat surprising coming from a prime minister who was himself the victim of religious discrimination – he was put in jail for reciting a religious poem.”