US Special Operations Forces got hold of a lot of documents during the raid on a ISIS Top Official in Syria in May this year. In some of those documents, It was clearly mentioned who can have sex with the female slaves.
Some of the rules include Father and Son can't have sexual relationship with the same woman slave. Even Owner of a Mother and Daughter weren't allowed to have sex with both. In addition, Joint Owners of a female slave are prohibited from intercourse.
When interviewed 20 female slaves who escaped the militants, It came to light that Islamic State fighters separated young women from old women & men. They would then be sold or presented as gifts after moving them in an organized manner from one place to the other. The slaves were repeatedly raped and brutally harassed in Syria and Iraq.