Sayyiduna Ali (may Allah be pleased with him) was asked about a man who had two slave-women who were sisters; he has intercourse with one of them: may he also have intercourse with the other? He said: "One verse makes them lawful while another makes them forbidden, and I have forbidden them to myself and to my children."
(b) A similar ruling is reported from Ibn Abbas (may Allah be pleased with them both) concerning a slave-woman and her daughter.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13936