The Sons of Your Father Through Breast-feeding From His Second Wife are Your Brothers

Question :

I was breast-fed by a woman, then her husband married. another woman and she bore him sons; are they brothers to me?

Answer:

If the breast-feeding took place five times or more and the milk was attributed to the husband by virtue of her having borne him children, then they are brothers for you from your father and your mother through breast-feeding. As for his sons from the other wife, they are brothers for you from your father due to breast-feeding.

Breast-feeding is when the child takes the breast and sucks milk until it reaches his stomach, then he stops breast-feeding, then returns and so on, until he has completed five or more whether it was in one sitting or more, and whether it was in one day or more, with the condition that it took place during the first two years of the child's life.

This is based upon the Hadith of the Prophet ﷺ:
"There is no breast-feeding except in the (first) two years."

And the Prophet ﷺ said Sahlah bint Suhail, may Allah be pleased with her:
"Breast-feed Salim five times and you will be forbidden to him."

And it is also based upon the Hadith confirmed in Sahih Muslim and Al-Jami [Sunan] At-Tirmithi on the authority of 'A'ishah, may Allah be pleased with her, in which she said: "It was revealed in the Qur'an that ten known breast-feedings caused prohibition, then it was abrogated by five known breast- feedings. Then the Prophet ﷺ died and the matter remained thus." and this wording is from the narration of At-Tirmithi. May Allah grant success to all in attaining what pleases Him.

Source:
Ibn Baz
Fatawa Islamiyah, Vol. 6 Pages 116-117