The daughter of a sister through breastfeeding is forbidden in marriage
Question

                My wife’s mother breastfed her son’s son (i.e., the son of my wife’s brother) more than five times, according to what my wife’s mother says (the one who breastfed him and is also his grandmother). Is this child then a brother to my wife through breastfeeding? Is he also a mahram for my daughter as a result and is it permissible for her to uncover in front of him?.
Praise be to Allah.Yes, this child has 
      become a brother to your wife through breastfeeding, so he is a maternal 
      uncle through breastfeeding to your children from this wife. So it is 
      permissible for your daughter – from this wife but not from any other wife 
      – to show in front of him what she shows in front of her mahrams. 

    The evidence for that is the fact that the Prophet  
    (peace and blessings of Allaah be upon him) said: “What becomes mahram 
    (forbidden for marriage) through breastfeeding is that which becomes mahram 
    through blood ties.”

    Narrated by al-Bukhaari, 2645; Muslim, 1447. 
And the daughter of a 
    sister through blood ties is a mahram, because Allaah says (interpretation 
    of the meaning): 
“Forbidden 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”

    [al-Nisa’ 4:23]

    So the daughter of your sister through breastfeeding is also 
    a mahram. 

    It says in Fataawa al-Lajnah al-Maa'idah, 21/116 

    The breastfeeding that makes a child a mahram is five or more 
    breastfeedings within the first two years. If the breastfeeding of the two 
    brothers was like that, then they are brothers through breastfeeding, and 
    the children of each of them are the children of the other through 
    breastfeeding, whether the milk belonged to the father and mother or to the 
    mother only, or to the father only. And it is not permissible for either of 
    them to marry the daughter of the other, because they are the daughters of 
    his brother through breastfeeding. 

    See also question no. 45819. 

    And Allaah knows best.