The children of your father through breastfeeding from his second wife who did not breastfeed you are siblings to you
Question

                I was breastfed by a woman, then her husband married a second wife, who produced children. Are they siblings to me?.
Praise be to Allah.
    If the breastfeeding consisted of five or more feedings and 
    the milk was attributed to the husband because she (the woman who breastfed 
    you) had had a child from him, then they are regarded as siblings to you 
    through your father and mother through breastfeeding. As for his children 
    from his second wife, they are siblings to you through your father through 
    breastfeeding. 

    What is meant by breastfeeding is when the child latches onto 
    the nipple and suckles milk until it reaches his stomach, then he let go of 
    the nipples for any reason, then he latches on again and suckles the nipple 
    until the milk reaches his stomach, then he stops breastfeeding, then he 
    breastfeeds again and so on, until he has breastfed five or more times, 
    whether that was in one sitting or more, and whether that occurred on one 
    day or more, on condition that that took place when the child was still in 
    the first two years of life, because the Prophet (blessings and peace of 
    Allah be upon him) said: “There is no breastfeeding (the makes a child a 
    mahram) except in the first two years.” And he (blessings and peace of Allah 
    be upon him) said to Sahlah bint Suhayl: “Breastfeed Saalim five times and 
    he will become your mahram.” 

    And it is proven in Saheeh Muslim and Jaami‘ 
    at-Tirmidhi that ‘Aa’ishah (may Allah be pleased with her) said: Among 
    the things that were revealed of the Qur’aan was that ten definite 
    breastfeedings make the child a mahram, then that was abrogated and replaced 
    with five definite breastfeedings, and the Messenger of Allaah (blessings 
    and peace of Allah be upon him) passed away when this was the case. This 
    version was narrated by at-Tirmidhi. 

    May Allah help us all to do that which pleases Him. End quote.