The daughters of a father through breastfeeding from his second wife are siblings of the child who was breastfed
Question

                My grandmother (my mother’s mother), may Allaah preserve her, breastfed her brothers and sisters from her father along with her own sons and daughters. After that, my grandfather (her husband) took another wife and she bore him a son and a daughter. Now my maternal aunts and my mother do not observe hijab in front of the sons of their maternal aunts and uncles because they regard themselves as paternal aunts. 

But my question is: Shouldn’t my grandfather’s daughter from his second wife observe hijab before my mother’s maternal uncles (the brothers of my grandmother whom she breastfed)?.
Praise be to Allah.
    Yes, your grandfather’s daughter from his second wife does 
    not have to observe hijab before your mother’s maternal uncles, because they 
    are her brothers through breastfeeding, but they are brothers on the 
    father’s side only. 

    The reason for that is: If a child is breastfed by a woman in 
    the manner mentioned in sharee’ah, i.e., within the first two years of life 
    and five or more full feedings, then he becomes the child of the woman who 
    breastfed him, and the daughters of the woman who breastfed him all become 
    his sisters. He also becomes a son of the one to whom the milk belongs, 
    namely her husband. The daughters of the husband all become sisters of the 
    child who is breastfed, whether they were born to the woman who breastfeeds 
    or from another woman, as is the view of the majority of scholars. This is 
    what is called the man’s milk. Al-Muntaqa by Shaykh al-Fawzaan. 

    Shaykh Ibn Baaz (may Allaah have mercy on him) was asked: I 
    was breastfed by a woman, then her husband took another wife, who bore him 
    sons; are they my brothers? 

    He replied: If there were five or more breastfeedings, and 
    the milk belonged to the husband because she bore him the child, then they 
    are your brothers through your father and mother through breastfeeding. As 
    for his children from his second wife, they are your brothers through your 
    father through breastfeeding.  

    Breastfeeding means that the child takes hold of the nipple 
    and sucks the milk until it reaches his stomach, then he lets go of the 
    nipple for any reason, then he suckles the nipple again until the milk 
    reaches his stomach, then he stops breastfeeding, then he resumes and so on, 
    until five feedings or more have taken place, whether that is in one sitting 
    or more, and whether that took place in one day or over several days, so 
    long as it was within the first two years of the child’s life, because the 
    Prophet  (peace and blessings of Allaah be upon him) said: “There is no 
    breastfeeding except in the first two years” and he  (peace and 
    blessings of Allaah be upon him) said to Sahlah bint Suhayl: ‘Breastfeed 
    Saalim five times and you will become a mahram for him.” And it is proven in
    Saheeh Muslim and Jaami’ al-Tirmidhi that ‘Aa’ishah (may 
    Allaah be pleased with her) 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, and the Prophet (peace and blessings 
    of Allaah be upon him) passed away when this was the case. This 
    version was narrated by al-Tirmidhi. May Allaah help us all to do that which 
    He loves and which pleases Him. End quote from Fataawa al-Shaykh Ibn 
    Baaz (22/305). 

    And Allaah knows best.