Do her children from breastfeeding have any relationship with her second husband?
Question

                A boy was breastfed by a woman along with her own son, then this woman got divorced and married another man, and had several sons and daughters. Her first husband married another woman and had several sons and daughters. 

My question is: 

I hope that you can tell me who are those who are regarded as the brothers and sisters of this child through breastfeeding – are they the sons and daughters of that woman through breastfeeding or the sons and daughters of the man? Please note that this woman says that she breastfed this boy for several days and her own son nursed from one breast and this boy nursed from the other.
Praise be to Allah.
    1 – When the infant has been breastfed by the woman five 
    times within the first two years of life, before weaning, then he becomes 
    her child through breastfeeding, according to the consensus of the imams, 
    and he becomes a mahram for her. The man to whom the milk belongs (because 
    he is the father of the child whose conception and delivery led to 
    production of the milk) becomes a father to the child through breastfeeding, 
    according to the consensus of the well-known scholars. This is proven 
    according to the Sunnah of the Messenger of Allaah  (peace and blessings 
    of Allaah be upon him). 

    2 – If the man and woman become parents of the child through 
    breastfeeding, then all their children become brothers and sisters to the 
    breastfed child, whether they are children of the father only (from another 
    wife) or of the mother only (from another husband) or from them both, or 
    they are their children through breastfeeding. They all become brothers and 
    sisters of this child through breastfeeding. Even if a man has two wives and 
    one breastfeeds a baby boy and the other breastfeeds a baby girl, these two 
    children become brother and sister and it is not permissible for them to 
    marry one another, according to the consensus of the four Imams and the 
    majority of Muslim scholars. Ibn ‘Abbaas was asked about this matter and 
    said: “The inseminator is one” – i.e., the man who had intercourse with both 
    women, which led to the pregnancies that produced the milk is one and the 
    same. 

    3 – According to Muslim consensus, there is no difference 
    between the children of the woman who were breastfed along with this child 
    and those who were born to her before or after that. 

    See Majmoo’ al-Fataawa, 24/31-32 

    4 – There is no paternal relationship between the woman’s 
    children through blood or through breastfeeding and her second husband, 
    i.e., he is not regarded as a father to them through breastfeeding. The milk 
    belongs to the first husband, who is a father to all of them. But her second 
    husband is their mother’s husband and by his marrying her, her daughters by 
    blood become mahrams to him, because they are his stepdaughters (rabaa’ib), 
    and when he has consummated the marriage with their mother they become 
    mahrams to him. Allaah says (interpretation of the meaning): 
“Forbidden to you (for 
    marriage) are: … your stepdaughters under your guardianship, born of your 
    wives to whom you have gone in”

    [al-Nisa’ 4:23]

    With regard to the relationship between the woman’s daughters 
    through breastfeeding and her second husband who was not the owner of the 
    milk, and whether or not they are mahrams for him because he is the husband 
    of their mother – this is a matter concerning which there is a difference of 
    opinion among the scholars. 

    The majority of scholars think that the woman’s daughters 
    through breastfeeding are mahrams for the second husband, because he is the 
    husband of their mother. Shaykh al-Islam Ibn Taymiyah was of the view, and 
    Shaykh Ibn ‘Uthaymeen agreed with him, that they are not mahrams for him. 

    Based on this, the women who were breastfed by this woman 
    should observe hijab in front of her second husband, because he is not a 
    mahram for them. 

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: If 
    we examine the view of the majority, which is that it is not permissible for 
    him to marry his wife’s daughter through breastfeeding and the view of 
    Shaykh al-Islam Ibn Taymiyah that she is not a mahram for him, we may draw 
    the conclusion that we should err on the side of caution because that is the 
    approach that is referred to in the Sunnah. Sa’d ibn Abi Waqqaas disputed 
    with ‘Abd ibn Zam’ah concerning a slave of Zam’ah. Sa’d said: “O Messenger 
    of Allaah, this is the son of my brother ‘Utbah ibn Abi Waqqaas, whom he 
    entrusted to my care because he is his son.” ‘Abd ibn Zam’ah said: “This is 
    my brother, the son of my father’s slave woman who was born on my father’s 
    bed.” The Prophet  (peace and blessings of Allaah be upon him) saw that 
    the child clearly resembled ‘Utbah but he said: “He is yours, O ‘Abd ibn 
    Zam’ah. The child belongs to the bed.” Then he said to Sawdah bint Zam’ah, 
    who was one of the Mothers of the Believers (the Prophet’s wives): “Observe 
    hijab from him, O Sawdah.” Al-Bukhaari, 2053; Muslim, 1457. Even though he 
    had ruled that he was a brother of hers, he told her to observe hijab from 
    him because he saw that he clearly resembled ‘Utbah. This was the ruling of 
    the Prophet  (peace and blessings of Allaah be upon him) that was based 
    on erring on the side of caution. He told her to observe hijab because there 
    was some doubt about him, and he ruled that he was her brother because he 
    was born in her father’s bed. 

    Duroos al-Haram al-Makki, vol. 3 
    p. 245. 

    And Allaah knows best. 

    For more information see question no.
    40226.