What is Forbidden Due to Fostering

Question :

There are two women, one of whom has a son and the other a daughter and it happened that they breast-fed.

each other's children. So who among the brothers of those who were breast-fed is permissible for the other? Advise us and may Allah reward you with goodness.

Answer:

If a woman breast-feeds a child five times or more during the first two years, the breast-fed child becomes a son for her, and for her husband, the owner of the milk. All of the woman's children from her husband, the owner of the milk and those from any other (previous or future husband) brothers.

of this foster child and the children of the husband, the owner of the milk from the breast-feeding woman or another become brothers of the foster child and her brothers become maternal uncles to him. The brothers of the husband, the owner of the milk, become paternal uncles to him, and the woman's father becomes a grandfather to the foster child, and her mother becomes a grandmother to the foster child, and the father of the husband, the owner of the milk, becomes a grandfather to the foster child, and his mother a grandmother to the foster child, according to the Words of Allah, Almighty, Most High in Surah An-Nisa', regarding those to whom marriage is forbidden:
"your foster mothers who suckled you, your foster milk suckling sisters."

-and the saying of the Prophet :
"What is unlawful by reason of lineage is unlawful by reason of fostering."

-and the saying of the Prophet:
"There is no fostering except within the first two years.

-and the authentic Hadith in Sahih Muslim-may Allah have mercy on him on the authority of 'A'ishah, may Allah be pleased with her, which states: "It was revealed in the Qur'an that breast-feeding ten times caused prohibition (of marriage), then it was abrogated and replaced by five times, then the Prophet died and the matter remained thus." Narrated by At- Tirmithi with this wording, and the essence of it is in Sahih Muslim.

Source:
Ibn Baz
Fatawa Islamiyah, Vol. 5 Pages 232-233