Can he marry the daughter of a woman who was breastfed by his mother?
Question

                A woman was breastfed by my mother twice, and the one who was breastfed alongside her was my older sister. I had not been born yet. The woman who had been breastfed by my mother got married, and she has a daughter. Is it permissible for me to marry her (the daughter)?.
Praise be to Allah.If a woman 
    breastfeeds a child, then this child becomes her child through radaa’ah 
    (breastfeeding), and a brother through breastfeeding of all her children, 
    whether those children were there before him or came later. 
Based on 
    this, any woman who was breastfed by your mother is your sister through 
    breastfeeding, and you are a maternal uncle to all her children. So it is 
    haraam for you to marry her daughter, because you are her maternal uncle 
    through breastfeeding. And the Prophet  (peace and blessings of Allaah 
    be upon him) said: “What becomes mahram (forbidden for marriage) through 
    breastfeeding is that which become mahram through blood ties.” Narrated by 
    al-Bukhaari, 2645; Muslim, 1447. 
It is haraam 
    for a maternal uncle through blood ties to marry his sister’s daughter, and 
    the same applies to the maternal uncle through breastfeeding. 
But it 
    should be noted that becoming a mahram through breastfeeding can only be 
    established through five well-known breastfeedings, because of the report 
    narrated by Muslim (1452) from ‘Aa’ishah who said: ““When the Qur’aan was 
    first revealed, the number of breast-feedings that would make a child a 
    relative (mahram) was ten, then this was abrogated and replaced with the 
    number of five which is well-known.” 
You state in 
    your question that this woman breastfed from your mother twice, so we should 
    note that the breastfeeding which makes the child a mahram is five 
    feedings. 
Ibn 
    al-Qayyim said in Zaad al-Ma’aad, 5/575: 
One 
    breastfeeding (rad’ah) means when the child takes the beast and starts 
    suckling, and then lets it go without being made to do so. That is one 
    breastfeeding, because the Lawgiver referred to one breastfeeding in general 
    terms, so it is to be interpreted according to custom, and this is the 
    custom. If the child stops briefly in order to breathe or to rest or because 
    of some distraction, then quickly goes back to the breast, this is regarded 
    as one breastfeeding, just as when a person who is eating pauses briefly 
    then quickly goes back to eating, this is not regarded as two meals, rather 
    it is one. This is the view of al-Shaafa’i.  And if the infant moves from 
    one breast to the other, this is one breastfeeding.  
See also 
    question no. 2864  
Thus it becomes clear 
    that the five breastfeedings could all take place in one sitting. 
If this 
    woman was breastfed by your mother five times in this sense, then it is not 
    permissible for you to marry her daughter, because you are her paternal 
    uncle through breastfeeding. If she was breastfed less than five times, that 
    does not mean that she is a mahram, so it is permissible for you to marry 
    her daughter. 
If there is 
    any doubt concerning the number of breastfeedings, whether it was five or 
    not, there is no proof of the child being a mahram in the case of doubt. 
Ibn Qudaamah 
    (may Allaah have mercy on him) said: 
If there is 
    any doubt as to whether breastfeeding took place or not, or with regard to 
    the number of breastfeedings, whether the number that makes the child a 
    mahram was completed or not, this does not make the child a mahram, because 
    the basic assumption is that he is not a mahram, and what is certain cannot 
    be dispelled by what is uncertain. 
See also 
    question no. 
    13357.
And Allaah 
    knows best.