She breastfed her sister’s daughter so that she would be a mahram to her sons
Question

                I breastfed my sister’s daughter with my sister’s agreement, and she breastfed my son at the same age, but that was without my husband’s knowledge, because I did not feel that there was anything wrong with that. I breastfed my sister’s daughter four times and my sister breastfed my son five times, because we wanted them to be siblings and to mix with one another with no reservation. When my husband found out about that, he told me that it is haraam, because we do not have any knowledge of the unseen. What is the ruling here? And what are the consequences of that? Thank you and may Allah reward you with all good.
Praise be to Allah.
      There is nothing wrong with you having breastfed your 
      sister’s daughter and your sister having breastfed your son with the aim 
      of making them mahrams, because this is a permissible aim and there may be 
      a need for that. It is not haraam, as your husband said. 

    There are two conditions for breastfeeding that establishes 
    the mahram relationship: 

    1.It should be 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, no. 1452.

    2.That should be within the first 
    two years of the child’s life, because of what is proven from the Prophet 
    (blessings and peace of Allah be upon him) in the hadeeth of ‘Abd-Allah ibn 
    al-Zubayr, that the Messenger of Allah (blessings and peace of Allah be upon 
    him) said: “There is no breastfeeding except that which fills the stomach.” 
    Narrated by Ibn Maajah (1946); see also Saheeh al-Jaami‘, no. 7495. 
    al-Bukhaari (may Allah have mercy on him) said in his Saheeh: 
    “Chapter: 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]”.

    Breastfeeding means that the infant takes the nipple and 
    suckles from it, then lets it go by himself, in order to breathe or move and 
    the like. 

    Based on that, the five breastfeedings may take place in one 
    sitting. 

    If your sister’s daughter breastfed from you five times, then 
    she has become your daughter through breastfeeding and a sister to all your 
    children, and to any other child who breastfed from you five times. 

    Similarly, if your son breastfed five times from his maternal 
    aunt, she has become a mother to him through breastfeeding and he has become 
    a brother to all of her children and any other child who breastfed from 
    her. 

    The mahram relationship is not limited to the girl and boy 
    asked about; rather the girl becomes a brother to all of your children, 
    present and future, and she is a daughter to your husband and a sister to 
    all of his children from any other wife, if applicable. 

    And Allah knows best.