Breastfeeding with the aim of establishing the mahram relationship
Question

                My wife is pregnant and so is her sister (and it seems that her baby is female). They are intending to deliberately breastfeed this daughter and my son – if Allah decrees that I should have a son – so as to avoid the issue of khulwah (being alone with a non-mahram member of the opposite sex) between female and male cousins. What is the ruling on that?.
Praise be to Allah.
    There is nothing 
    wrong with your wife breastfeeding her sister’s daughter and her sister 
    breastfeeding her son. If there are five breastfeedings and that is done in 
    the first two years of life, the girl will become a sister through 
    breastfeeding to all of your and your wife’s children, and your son will 
    become a brother through breastfeeding to all of the children of his 
    maternal aunt and her husband. 

    The breastfeeding 
    that establishes the mahram relationship must meet two conditions: 

    1.

    That there should 
    have been five or more breastfeedings, because of the hadeeth of ‘Aa’ishah 
    (may Allah be pleased with her), who said: Among the things that were 
    revealed of the Qur’aan was that ten definite breastfeedings make a person a 
    mahram, then that was abrogated and replaced with five definite 
    breastfeedings

    Narrated by 
    Muslim, 1452 

    2.

    That should be 
    within the first two years of the child’s life, because it was proven that 
    the Prophet (blessings and peace of Allah be upon him) said: “There is no 
    breastfeeding except that which fills the stomach.”

    Narrated by Ibn 
    Maajah, 1946; also in Saheeh al-Jaami‘, no. 7495 

    Al-Bukhaari (may 
    Allah have mercy on him) said in his Saheeh: Chapter on one who says 
    there is no breastfeeding after two years because Allah says (interpretation 
    of the meaning): “The mothers shall give suck to their children for two 
    whole years, (that is) for those (parents) who desire to complete the term 
    of suckling” [al-Baqarah 2:233]. 

    There is nothing 
    wrong with breastfeeding children with the aim of establishing the mahram 
    relationship between them, because this is a permissible aim and there may 
    be a need for it. The Messenger (blessings and peace of Allah be upon him) 
    instructed the wife of Abu Hudhayfah to breastfeed Saalim so that he would 
    become a mahram to her. 

    And Allah knows 
    best.