Would the paternal aunt who breastfed her daughter's son be considered a prohibitive relation to her brother's daughters?

If your grandfather's second wife breastfed her daughter's son five times with your grandfather's milk, then he becomes a mahram to you, and it is permissible for him to see from you what is permissible for mahrams. However, if the suckling is less than five feedings, then the prohibition is not established. If she breastfed him with the milk of another husband, then he would not be a mahram to you according to the Shafi'is, Hanafis, and Hanbalis with some detail, but he would be a mahram according to the Malikis.