But my question is: Shouldn’t my grandfather’s daughter from his second wife observe hijab before my mother’s maternal uncles (the brothers of my grandmother whom she 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.