Allah said,

﴿وَرَبَائِبُكُمُ اللَّـتِى فِى حُجُورِكُمْ﴾

(...your stepdaughters under your guardianship,) The majority of scholars state that the stepdaughter is prohibited in marriage for her stepfather ﴿who consummated his marriage to her mother﴾ whether she was under his guardianship or not. The Two Sahih recorded that Umm Habibah said, "O Messenger of Allah! Marry my sister, the daughter of Abu Sufyan (and in one narration `Azzah bint Abu Sufyan).'' He said,

«أَوَ تُحِبِّينَ ذلِك»

؟ قالت: نعم. لست لك بمخلية، وأحب من شاركني في خيرٍ أختي، قال:

«فَإِنَّ ذلِكِ لَا يَحِلُّ لِي»

. قالت: فإنا نتحدث أنك تريد أن تنكح بنت أبي سلمة، قال:

«بِنْتَ أُمِّ سَلَمَة»

؟ قالت: نعم. قال:

«إِنَّهَا لَوْ لَمْ تَكُنْ رَبِيبَتِي فِي حِجْرِي مَا حَلَّتْ لِي، إِنَّهَا لَبِنْتُ أَخِي مِنَ الرَّضَاعَةِ، أَرْضَعَتْنِي وَأَبَا سَلَمَةَ ثُوَيْبَةُ، فَلَا تَعْرِضْنَ عَلَيَّ بَنَاتِكُنَّ وَلَا أَخَوَاتِكُن»

("Do you like that I do that'' She said, "I would not give you up for anything, but the best of whom I like to share with me in that which is righteously good, is my sister.'' He said, "That is not allowed for me.'' She said, "We were told that you want to marry the daughter of Abu Salamah.'' He asked, "The daughter of Umm Salamah'' She said, "Yes.'' He said, "Even if she was not my stepdaughter and under my guardianship, she is still not allowed for me because she is my niece from suckling, for Thuwaybah suckled me and Abu Salamah. Therefore, do not offer me to marry your daughters or sisters. '') In another narration from Al-Bukhari,

«إِنِّي لَوْ لَمْ أَتَزَوَّجْ أُمَّ سَلَمَةَ مَا حَلَّتْ لِي»

(Had I not married Umm Salamah, her daughter would not have been allowed for me anyway.) Consequently, the Messenger stated that his marriage to Umm Salamah was the real reason behind that prohibition.