Must the granddaughter, from the grandfather's second wife, observe hijab in front of her maternal uncles (the grandmother's brothers whom she breastfed)?

Yes, it is permissible for your grandfather's daughter from his second wife not to wear a hijab in front of your mother's maternal uncles because they are her siblings through suckling. If a child suckles from a woman five times or more within the two years of infancy, he becomes a son to the wet nurse and her husband (the owner of the milk). Consequently, all the husband's daughters become sisters to this suckled child. Therefore, the children of the wet-nursing husband's second wife are considered siblings to the suckled child through his father by suckling.