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

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.