A woman got married to a married man who has sons and daughters. The question is: Are the husbands of the step-daughters of this woman permanently unlawful for her? Please advise. May Allaah Reward you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

The husbands of the daughters of this man are not permanently or even temporarily unlawful for his other wives, for they are not among these female prohibited consanguinities whom Allaah Almighty Mentioned in the Surah of An-Nisaa’ in the verse (which means): {Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your father's sisters, your mother's sisters, your brother's daughters, your sister's daughters, your [milk] mothers who nursed you, your sisters through nursing, your wives' mothers, and your step-daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allaah Is ever Forgiving and Merciful.}[Quran 4:23]

Al-Qurtubi said in his Tafseer:

In this verse, Allaah Almighty Mentioned the lawful and unlawful of women (for marriage), including the prohibition of the father's wife. Thus, seven women of blood-relations, and six of relations by suckling and marriage were prohibited (for marriage), and a seventh was joined with them by a collectively contiguous Sunnah, i.e. to combine a woman and her paternal aunt in a wedlock. This was stated by consensus. It was also narrated on the authority of Ibn ‘Abbaas that he said, "Seven (women) of blood-relations, and other seven of relations by marriage have been made unlawful (for marriage)." Then, he recited the previous verse. The same was stated by ‘Amr ibn Saalim, a freed slave belonging to the Ansaar. He said, "And the seventh is included in the Statement of Allaah Almighty (which means): {And [also prohibited to you are all] married women...}[Quran 4:24]"

But if anyone of the husbands of those daughters has a suckling relation with the father's other wife, she then will be permanently unlawful for him (in marriage), like her being his suckling mother or sister. "Indeed, suckling relation prohibits what corresponding blood-relation prohibits." [Al-Bukhari, Ahmad, along with Abu Daawood, At-Tirmithi, An-Nasaa’i and Ibn Maajah]