Is it permissible for the sons of a brother to marry the daughter of a sister who was breastfed by their mother (her grandmother) for nine months?

If the suckling occurred within the two years [of infancy], then it is not permissible for your brothers' children to marry your brother's daughter, because she has become their sister through suckling, and consequently, an aunt to their children through suckling. The proof for this is the Prophet's (peace and blessings be upon him) saying: "What is forbidden by lineage is forbidden by suckling."