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

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.