Their father’s wife breastfed a baby girl – is she regarded as their sister?
Question

                My father married a woman in the past and was blessed with children from this wife. This woman died after breastfeeding another girl who was not her relative; rather she did that as an act of kindness. Are we brothers of this girl? Her father will not allow her to uncover in front of us unless there is a fatwa.
Praise be to Allah.If this 
    woman breastfed the child for five known breastfeedings during the first two 
    years of life, then she is a daughter of the woman and her husband who is 
    the “owner of the milk” – namely your father – and all the children of that 
    woman from her husband, the owner of the milk, or from any other husband 
    will be brothers and sisters of that girl; the children of the husband, the 
    owner of the milk, from the woman who nursed the infant or from any other 
    wife, will also be brothers and sisters of that girl. 
So all of 
    you, siblings born to your father from any of his wives, are all brothers 
    and sisters of that girl.  
Shaykh Ibn 
    Baaz (may Allaah have mercy on him) said: “If a woman breastfeeds a child 
    for five or more known breastfeedings within the first two years, that child 
    becomes the son or daughter of her and her husband, the owner of the milk, 
    and all the children of that woman, from her husband who is the owner of the 
    milk or any other husband, become brothers and sisters of this infant. The 
    children of the husband who is the owner of the milk, from the wife who 
    breastfed the child or any other wife also become brothers and sisters of 
    the infant. Her brothers become maternal uncles for him, and the brothers of 
    the husband who is the owner of the milk become paternal uncles. The father 
    of the woman becomes a grandfather for the child, as does the father of the 
    husband who is the owner of the milk, and his mother becomes a grandmother 
    to the child, because Allaah says concerning female mahrams in Soorat 
    al-Nisa’ (interpretation of the meaning): 
“your 
    foster mothers who gave you suck, your foster milk suckling sisters”
[al-Nisa’ 
    4:23]
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.” And he said: “There is no breastfeeding (that makes the 
    child a mahram) except in the first two years.” And it was proven in 
    Saheeh Muslim 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; the original is in Saheeh Muslim. 
From Fataawa 
    Islamiyyah, 3/333 
With regard to the 
    definition of breastfeeding, please see question no,
    27280.