The children of your father through breastfeeding from his second wife who did not breastfeed you are siblings to you

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.