The group is made up of 16 activists in Raqqa who work to shed light on the jihadists' actions in the city.
The Telegraph is not able to independently verify their claims, but they have a good track record for accuracy in their reporting.
According to a report published on the group's website, the Isis source said that after the Yazidi women were captured in Iraq they were divided up, separating out "virgins" from married women and mothers.
The virgins were then distributed among "Isis soldiers" who had taken part in the battle in Sinjar province.
The virgin Yazidi women and girls were reportedly made to convert to Islam and then forced to marry the jihadists.
"After marrying her [and using her for sex], he might decide to divorce her and pass her on to another fighter," the source said.
Some of the women were brought to Raqqa province and given as gifts to the leaders of Isis, the source added.
The Isis emirs - leaders - then immediately took the girls out of Raqqa, instead keeping them in smaller towns in the northern Syrian countryside, such as Tal Abyad and Ain Issa, so that they would remain hidden.
"The inner circle of Isis leaders and security officials were careful that [the details] of this issue should not be known" by Raqqa civilians, the group said.
Raqqa is Being Slaughtered Silently had previously been wary of rumours of the practice of sex slavery, often writing them off as untrue propaganda.
But the group's Isis source confirmed the use of Yazidi women for sex by their captors, with money changing hands.
The report adds further weight to the admission in the Dabiq publication and supporting reports by international rights NGOs including Human Rights Watch, which have interviewed witnesses and relatives of women who were kidnapped by Isis.
Who are the Yazidis
• They are a pre-Islamic Kurdish sect who live in northern Iraq, Syria and Turkey.
• They say they have often faced persecution in Iraq because the chief angel they venerate as a manifestation of God is often identified as the fallen angel Satan in biblical terminology.
• The Yazidi religion is a syncretic combination of Zoroastrian, Manichaean, Jewish, Nestorian Christian and Islamic faiths.
• The Yazidi themselves are thought to be descended from supporters of the Umayyad caliph Yazid I.
• They believe that they were created separately from the rest of mankind, not even being descended from Adam, and they have kept themselves strictly segregated from the people among whom they live.
