Does the paternal cousin who was breastfed by the grandmother become a forbidden degree of relation (mahram) to the questioner's sisters and to the daughters of her paternal uncles and aunts, given that he has become a brother to her father and an paternal uncle to her through breastfeeding?

It is not permissible for your cousin to marry the daughters of his paternal uncles or paternal aunts if he was nursed by their grandmother, because his paternal uncles and paternal aunts become his brothers and sisters through nursing, and their daughters become the daughters of his brothers through nursing. Indeed, marriage to the daughters of brothers and sisters, whether by lineage or by nursing, has been prohibited.