If his stepdaughter has a daughter through breastfeeding, is he a mahram to her?
Question

If the wife’s daughter has a daughter through breastfeeding, is the mother’s husband a mahram to this daughter?
Praise be to Allah.If a man marries a woman and consummates the marriage with her, if she has a daughter from another man, who is a stepdaughter (rabibah), this stepdaughter becomes a mahram for him [meaning that she is forbidden for him to marry] according to the text of the verse.
Allah, may He be exalted, says (interpretation of the meaning):
{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} [an-Nisa’ 4:23].
This prohibition also includes the daughter of this stepdaughter.
Ibn al-Qayyim (may Allah have mercy on him) said:
Stepdaughters who are under the guardianship of husbands are prohibited for marriage. They are the daughters of their wives with whom they have consummated marriage. That also includes their daughters, their daughters’ daughters and sons’ daughters; they are all included in the word stepdaughters (raba’ib)."(Zad al-Ma‘ad 5/168).
It says in al-Mawsu‘ah al-Fiqhiyyah al-Kuwaitiyyah (36/216):
Raba’ib (stepdaughters) is the plural of rabibah. The stepchild of a man is the child of his wife from another man. The stepchild is called his rabib because he is the one who raises him (yurubbuhu), meaning that he takes care of him. The stepdaughter (rabibah) is the daughter of the wife, and she is prohibited for her mother’s husband to marry according to the verse, whether she is under his guardianship or not, and she is looked after like his own daughter in terms of compassion and care. With regard to the daughters of the stepdaughter and the daughters of the stepson being mahrams [and prohibited for marriage], that is established by scholarly consensus. End quote.
For more information, please see the answer to question no. 163263 .
As the stepdaughter becomes a mahram to her mother’s husband because of ties through marriage, the daughter of the stepdaughter becomes a mahram to him because of lineage and the ties of kinship between her and her mother.
She also becomes a mahram to him if motherhood is not because of lineage and giving birth, rather it is because of breastfeeding, because of the general meaning of the text which indicates that what becomes mahram (forbidden for marriage) through breastfeeding is the same as that which becomes mahram through ties of kinship.
According to the hadith, Ibn ‘Abbas (may Allah be pleased with him) said: The Prophet (blessings and peace of Allah be upon him) said regarding the daughter of Hamzah: “She is not permissible for me (to marry), for what becomes mahram (forbidden for marriage) through breastfeeding is the same as that which becomes mahram through lineage, and she is the daughter of my brother through breastfeeding.” Narrated by al-Bukhari (2645) and Muslim (1447).
Al-Bukhari (5099) and Muslim (1444) narrated from ‘Amrah bint ‘Abd ar-Rahman that ‘A’ishah, the wife of the Prophet (blessings and peace of Allah be upon him), told her that the Messenger of Allah (blessings and peace of Allah be upon him) was in her house, and she heard the voice of a man asking permission to enter Hafsah’s house. ‘A’ishah said: I said: O Messenger of Allah, there is a man asking permission to enter your house. The Messenger of Allah (blessings and peace of Allah be upon him) said: “I think it is So and so” – Hafsah’s paternal uncle through breastfeeding. ‘A’ishah said: O Messenger of Allah, if So and so – her paternal uncle through breastfeeding – were still alive, could he enter my house? The Messenger of Allah (blessings and peace of Allah be upon him) said: “Yes, for what becomes mahram (forbidden for marriage) through breastfeeding is the same as that which becomes mahram through birth.”
The view that breastfeeding is equal to lineage when there is a connection through marriage is the view of the majority of scholars.
Ibn Rajab (may Allah have mercy on him) said:
The prohibition on marriage to a mahram through breastfeeding extends as far as the same prohibition through lineage when there is a connection through marriage, either with regard to the man’s lineage, such as the wife of his father or son, or with regard to the woman’s lineage, such as her mother and daughter, and with regard to women to whom it is prohibited to be married at the same time because of the women’s lineage also, such as being married to two sisters, or to a woman and her paternal aunt or her maternal aunt, at the same time. All of that becomes prohibited through breastfeeding as it becomes prohibited through lineage, because it is included in the Prophet’s words, “what becomes mahram (forbidden for marriage) through breastfeeding is the same as that which becomes mahram through lineage.”
All of that becomes prohibited because of lineage, and some of it is because of the husband’s lineage and some of it is because of the wife’s lineage. This was stated by the leading scholars of the early generations, and no difference of opinion is known to have occurred among them concerning that. It was also stated by Imam Ahmad, who quoted as evidence the general meaning of the Prophet’s words, “what becomes mahram (forbidden for marriage) through breastfeeding is the same as that which becomes mahram through lineage.”(Jami‘ al-‘Uloom wa’l-Hikam  2/442).
Thus the mahram relationship may be established through breastfeeding as it may also be established through lineage.
Al-Hafiz Ibn Hajar (may Allah have mercy on him) said:
The Prophet’s words, “Breastfeeding makes prohibited what birth makes prohibited” imply that breastfeeding also makes permissible what birth makes permissible, according to scholarly consensus. This has to do with the prohibition on marriage and other issues connected to it. The mahram relationship with the one who was breastfed includes the children of the woman who breastfed him, and they are to be viewed as being like close relatives with regard to it being permissible to look at them, be alone with them and travel with them, but that does not include other rulings on motherhood such as inheritance, the obligation to spend on maintenance, the obligation to manumit a relative if one finds him enslaved and buys him, and rulings on giving testimony, paying blood money and waiving retaliatory punishments (qisas)."(Fath al-Bari (9/141).
Shaykh al-Islam had a different view concerning that. He favoured the view that it does not make him a mahram. He said in al-Ikhtiyarat (p.305):
The daughter of a stepdaughter is a mahram (prohibited for marriage) because she is a stepdaughter.
He also said in al-Ikhtiyarat (p. 308).
The mahram relationship that is established through marriage cannot be established on the basis of breastfeeding, so it does not become prohibited for a man to marry his wife’s mother or daughter through breastfeeding. End quote.
In ash-Sharh al-Mumti‘ (12/124), Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) stated that the view of Ibn Taymiyah is more likely to be correct, but he took a prudent approach and said that the view of the majority should be followed.
And Allah knows best.