Is it permissible for a brother to marry his paternal uncle's daughter if the paternal uncle's wife breastfed the prospective husband's sister along with her elder son, knowing that the mother did not breastfeed her own children, and that the brother is older than his nursed sister, and the paternal uncle's daughter is younger than her brother?

Your brother is permitted to marry his paternal uncle's daughter, provided there is no breastfeeding relationship between them. This means neither of them was suckled by the mother of the other, nor by their father's wife other than their own mother, nor did they share suckling from another woman. The breastfeeding of his sister with her brother does not prohibit her to him. The breastfeeding that establishes a prohibited degree of kinship is five full sucklings.