His mother breastfed his maternal aunt; what is the ruling on marrying the daughters of his maternal aunt?
Question

                My brother and I want to marry the daughters of my maternal aunt who is younger than my mother, but we heard that my mother’s mother breastfed my brother, and my mother and my maternal aunt breastfed my younger aunt, and we are worried about the mahram relationship in this situation. What is the shar‘i ruling in this case? Is it permissible for me to marry the daughter of my maternal aunt and is it permissible for my brother to marry her younger sister?.
Praise be to Allah.
    What we understand from your question is that your brother 
    was breastfed by your grandmother and that your younger maternal aunt was 
    breastfed by your mother and your older maternal aunt. 

    If what we understand is what was meant, and your brother 
    breastfed from your grandmother for five complete feedings before he reached 
    the age of two years, then your brother has become a brother to your mother 
    and maternal aunts. Based on that, it is not permissible for him to marry 
    the daughter of your maternal aunt, because he has become a maternal uncle 
    to her through breastfeeding. 

    If your younger maternal aunt was breastfed by your mother 
    for five complete feedings in the first two years of life, then she has 
    become a sister to you and your brother, and you have become maternal uncles 
    to her daughters. 

    Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: If 
    the child was breastfed by his maternal grandmother for five or more 
    complete feedings within the first two years of life, he thus becomes a 
    brother to his maternal uncles and aunts, and a paternal uncle to the 
    children of his maternal uncles, and a maternal uncle to the children of his 
    maternal aunts. So it is not permissible for him to marry any of the 
    daughters of his maternal uncles, because he became a paternal uncle to them 
    through breastfeeding. And it is not permissible for him to marry any of the 
    daughters of his maternal aunts, because he became a maternal uncle to them 
    through breastfeeding. And Allah is the source of strength. 

    End quote from Majmoo‘ Fataawa Ibn Baaz, 22/306 

    Based on that, it is not permissible for you and your brother 
    to marry the daughters of your maternal aunt, because you have become 
    maternal uncles to them. 

    And Allah knows best.