If a sister breastfeeds her brother’s son, does marriage between her son and her brother’s daughter become forbidden? And does her son become a brother to any new daughter her brother is blessed with?

If your nephew suckles from you, he becomes your son through fosterage and a brother to your children. As for the rest of your brother's children who did not suckle from you, they are not considered your children through fosterage, and your son is permitted to marry them.