What did the Prophet Muhammad (saw) say regarding the legality of marrying two sisters at the same time?
 Narrated Um Habiba:                     I said, "O Allah's Apostle! Marry my sister, the daughter of Abu Sufyan." He said, "Do you like that?" I said, "Yes, for even now I am not your only wife; and the most beloved person to share the good with me is my sister." The Prophet said, "But that is not lawful for me (i.e., to be married to two sisters at a time.)" I said, "O Allah's Apostle! By Allah, we have heard that you want to marry Durra, the daughter of Abu Salama." He said, "You mean the daughter of Um Salama?" I said, "Yes." He said, "By Allah ! Even if she were not my stepdaughter, she would not be lawful for me to marry, for she is my foster niece, for Thuwaiba has suckled me and Abu Salama; so you should neither present your daughters, nor your sisters to me."