Most of his maternal cousins were breastfed by his mother and he was breastfed by his maternal aunt; what is the ruling on his brother?
Question

                I was breastfed by my maternal aunt and my mother breastfed most of her children. Is it permissible for my brother to marry the daughter of my maternal aunt, knowing that he was not breastfed by my maternal aunt?.
Praise be to Allah.
    If a person is breastfed by a woman for five definite 
    feedings, he becomes her son through breastfeeding and a brother to all her 
    children, both males and females. 

    The evidence for that is the reports narrated by Muslim 
    (1452) from ‘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.  

    The definition of breastfeeding is as Ibn al-Qayyim (may 
    Allah have mercy on him) 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. End quote. 

    Based on that, if you were breastfed by your maternal aunt 
    five times, then you are the brother of all her sons and daughters, and it 
    is not permissible to you to marry one of them. 

    The ruling on breastfeeding has to do with the person who 
    breastfed and his children. As for his siblings, the ruling on breastfeeding 
    does not include them. 

    Based on that, if the daughter of your maternal aunt whom 
    your brother wants to marry breastfed from your mother, then she is his 
    sister through breastfeeding and it is not permissible for him to marry her. 
    But if she was not breastfed from your mother, then there is nothing wrong 
    with them getting married, because there is no impediment to their doing so 
    and that is not affected by the fact that you were breastfed by her mother 
    or that her siblings were breastfed by your mother. 

    And Allah knows best.