If the breastfeeding consisted of five or more feedings and the milk was attributed to the husband because she (the woman who breastfed you) had had a child from him, then they are regarded as siblings to you through your father and mother through breastfeeding. As for his children from his second wife, they are siblings to you through your father through breastfeeding.
What is meant by breastfeeding is when the child latches onto the nipple and suckles milk until it reaches his stomach, then he let go of the nipples for any reason, then he latches on again and suckles the nipple until the milk reaches his stomach, then he stops breastfeeding, then he breastfeeds again and so on, until he has breastfed five or more times, whether that was in one sitting or more, and whether that occurred on one day or more, on condition that that took place when the child was still in the first two years of life, because the Prophet (blessings and peace of Allah be upon him) said: There is no breastfeeding (the makes a child a mahram) except in the first two years. And he (blessings and peace of Allah be upon him) said to Sahlah bint Suhayl: Breastfeed Saalim five times and he will become your mahram.
And it is proven in Saheeh Muslim and Jaami at-Tirmidhi that Aa'ishah (may Allah be pleased with her) said: Among the things that were revealed of the Qur'aan was that ten definite breastfeedings make the child a mahram, then that was abrogated and replaced with five definite breastfeedings, and the Messenger of Allaah (blessings and peace of Allah be upon him) passed away when this was the case. This version was narrated by at-Tirmidhi.
May Allah help us all to do that which pleases Him. End quote.
Reproduced from Islam QA