The daughters of a father through breastfeeding from his second wife are siblings of the child who was breastfed

Yes, your grandfather’s daughter from his second wife does
not have to observe hijab before your mother’s maternal uncles, because they
are her brothers through breastfeeding, but they are brothers on the
father’s side only. 

The reason for that is: If a child is breastfed by a woman in
the manner mentioned in sharee’ah, i.e., within the first two years of life
and five or more full feedings, then he becomes the child of the woman who
breastfed him, and the daughters of the woman who breastfed him all become
his sisters. He also becomes a son of the one to whom the milk belongs,
namely her husband. The daughters of the husband all become sisters of the
child who is breastfed, whether they were born to the woman who breastfeeds
or from another woman, as is the view of the majority of scholars. This is
what is called the man’s milk. Al-Muntaqa by Shaykh al-Fawzaan. 

Shaykh Ibn Baaz (may Allaah have mercy on him) was asked: I
was breastfed by a woman, then her husband took another wife, who bore him
sons; are they my brothers? 

He replied: If there were five or more breastfeedings, and
the milk belonged to the husband because she bore him the child, then they
are your brothers through your father and mother through breastfeeding. As
for his children from his second wife, they are your brothers through your
father through breastfeeding.  

Breastfeeding means that the child takes hold of the nipple
and sucks the milk until it reaches his stomach, then he lets go of the
nipple for any reason, then he suckles the nipple again until the milk
reaches his stomach, then he stops breastfeeding, then he resumes and so on,
until five feedings or more have taken place, whether that is in one sitting
or more, and whether that took place in one day or over several days, so
long as it was within the first two years of the child’s life, because the
Prophet (peace and blessings of Allaah be upon him) said: “There is no
breastfeeding except in the first two years” and he (peace and
blessings of Allaah be upon him) said to Sahlah bint Suhayl: ‘Breastfeed
Saalim five times and you will become a mahram for him.” And it is proven in
Saheeh Muslim and Jaami’ al-Tirmidhi 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. May Allaah help us all to do that which
He loves and which pleases Him. End quote from Fataawa al-Shaykh Ibn
Baaz (22/305). 

And Allaah knows best.