What is the ruling regarding a master having sexual intercourse with two sister-slaves simultaneously?

Primary text

If a master has intercourse with two sister-slaves at the same time, the intercourse with the second sister is prohibited (haram), but no legal penalty (hadd) is applied. This is because the intercourse occurred while both were his property, and the status of the second sister is disputed (i.e., whether the first act made her illicit). Furthermore, the master has a potential path to make the union lawful (istibaha). This situation differs from having intercourse with a legally owned sister by fosterage. The master is forbidden from having intercourse with either sister until the other is rendered illicit and has completed her waiting period (istibra'). The supporting argument is that the second sister has become a marital bed (firash) to him, meaning the paternity of any resulting child would be attributed to him, thus prohibiting her sister, similar to if he had initiated intercourse with her in the first place. The counterargument that unlawful intercourse does not prohibit lawful relations is unsubstantiated and is contradicted by cases where intercourse during menstruation, post-natal bleeding (nifas), or Ihram renders the sister, mother, or daughter permanently unlawful.